So, based upon the Hannah Glasse recipe that
steb found, it rather looks like the "old tymey" way of making Yorkshire Pudding was never the "old tymey" way of making Yorkshire pudding.
Modern way: Hot oven (probably hotter than the position in the fireplace picked by HG on a given day), sealed, basically (certainly compared to a fireplace without even lids to the vessels used) allowing heat to be constant above and below, as well as allowing humidity to build to a higher level, heated oil to begin with, the watery "batter" (compared to a less watery by far "dough"), and a cooking time that waits on the cooking of the batter into a bread-y result. At some point around here, one takes them out and eventually makes use of them, as such pretty much, with some (modern) gravy and meat, usually on them, but conceivably as dipping materials or put-them-together-and-get-the-fork-to-your-mouth kind of arrangement.
Alternate result: end up with popovers.
HG's recipe differs in a couple respects. The humidity will NEVER build up the way it can in a modern, pretty sealed, oven. So the rising will be much less. Why? There are no rising agents AT ALL in either recipe, none. The only thing to make the bread expand during cooking is the moisture in the batter. At the moment, think ball of choco-chip cookie dough but runnier so already flattened out, largely. What forces them into the unusual shapes? The exceedingly hot oil the batter is placed upon begins making a bottom crust immediately, and heats the rest. That rest expands every direction, but the underneath is already crusting so wherever it expands there will be a tendency to honor that crusting, so to speak, limiting it from running everywhere. A little more here, a little less there, the batter above the slightly hardening crust underneath it expanding up and about as it all happens... we get the unique shaping of puddings even in the same pan. But it has to have immediate heating to the batter, not just the crust so as to start evaporation and maybe even some micro-boiling, to push the upper layer of it all up. Since it can spread it doesn't have to end up straight up like a popover, but more, with the outer top hardening as well, slowly, outward becomes prevalent, not "runaway" but prevalent. That is accentuated in the fireplace cooking as the crusting above is not fought by a rising humidity carrying some moisture the top crust softening it and letting the rise be straight up. The hardest part of the top? The original top of the batter. That also inhibits the up rising since pushing sideways pushes against less set/hard crust.
And that's how we leave it (or "leave well enough alone"... no value judgment here).
In the HG recipe, that is not what happens. Once it is "nigh enough" one cuts off the intensity of the heat from below so as to not burn the oil (its THAT hot underneath) and therefore the pudding. We take it out and end things. She continues, and is essentially addressing the modern oven humidity increase due to being sealed. In her recipe, the physicality is arranged to immediately begin dripping meat juices ("gravy" in the whole western world at the time, though in about three(+) generations Escoffier would formalize "sauces" and what was meant by "gravy" would forever change) on the top crust (softening it... and unlike our humidity, adding flavor!). And when nicely browned, she dried them rather than letting their crusting and internals soften while sitting. Even then, she layers on the flavor, using the butter as a dipper, like we might an au jus, the "gravy of the meat" either referring to the flavoring done letting the meat drip on them after reaching "nigh enough" or possibly also having an au jus nature by ladling more drippings on as a definite step. (I know I would, but I think this less likely for her meaning. There is no direct statement of it and it would have been easy for there to be.)
So, the major difference would be that we do not flavor in the last portion of the cooking. (Which can't be only a couple of minutes long, since meat takes some time to drip enough to be useful as a softener while still cooking, or as a flavorant. It must be a stage at least 5-10 minutes long. "Nigh enough" must mean when a crust begins really existing on the top as it would just run off or puddle otherwise.) The oven humidity would keep the top crust just slightly pliable rather than hardened and hardening more so we get more of a croissant effect than a rock-like crust. For her, the drippings achieve this and flavor at the same time. Her cup of butter, clearly for dipping (so treating the pudding as a flatbread), makes it clear the pudding is meant to be a MOIST mouthful, and she expects it to be dry enough internally to need the butter to make that so. We don't address that unless eating the pudding before "resting" evaporation dries it out even more. Yet she isn't simply drenching it, but providing the butter dip to moisten as needed and to preference more so than to taste. Taste would probably be provided by the meat being included with each mouthful, topped off, so to speak, by whatever amount of butter was added in the dipping.
Now, that leads to a "what do you want to achieve by using the 'old tymey' method you have always been told existed" situation. (Yes, no one was ever explicit, probably, in saying meat drippings provided ALL the oily liquid used, but they are clearly implying, to the extent that even using meat drippings for the oily material underneath the batter that is heated so high one has to eventually take a measure to keep it no hotter so it won't burn, that the oft-heard "you can even use meat drippings for the hot oil you drop the batter into" means "Why not?" rather than anything else particularly the impression of adding a flavor layer, rather than its characteristics being needed to set the times when you perform the steps and change the cooking. Modern baking may allow for shifting trays to even out heating of batches, but surely never says anything like "when the cookie dough stops spreading immediately lower the heat"... you make so few of that kind of adjustment even to heat and it usually doesn't matter since the heat takes so long to fall off, and you do it at times, not at marker points in the product's baking process. (Something to think about doing rather than slavishly changing a temperature at "11 minutes"...)
So if you really desire to have a result more like the folks in 1800 or maybe 1600 might have had, you need to figure how to, when the top crust reaches a tipover point where it can absorb drippings, begin to provide drippings. Removing the humidity won't hurt if you provide the drippings. How? That's a trick I guess. A modern oven would be a hard proposition for both applying drippings directly from the cooking meat, and keeping the puddings baking at a higher temperature than the meat is. If your oven is convection, that could be both helpful and not. Drippings are two materials: fat rendering out of the meat, and liquids being squeezed out of its cells by the heating proteins shrinking, thereby shrinking the cells. You could fry up a fat pad from a roast, beef or especially pork as it's often provided with more fat in the market as pork has so much fat wrapping it to begin with) — carefully slice it horizontally to a thickness experience would dictate, maybe begin at less than 1/8th inch thick, or as thin as you can safely achieve (perhaps easier if prepare it beforehand and make sheets when cold and stiffer) or just get some fat nets available some places and apply enough to achieve... enough... I feel like a 1700's recipe writer saying that!... but heated before application — so it'd be hot and rendering when needed, then open the oven and place portions on each pudding's top. Maybe paint each quickly with an au jus from pan drippings from the several preceding days, then apply. Maybe someone would invent a product if this would take off?
Even better might be cooking bacon to the point of almost rendering or just beyond, if one works immediately rather than "multi-tasking", still pliable, and layering it onto the puddings... fats AND meat liquids... that might be the very thing. Fatback might be going too far the fat direction (just using in the previous paragraph as "the best it seems one could do"), but "to taste" varies for every family and in families too.
Then, no matter how you've physically used Yorkshire puddings in your house in the past, use them as a flatbread. When they are ready as the meal comes together should likely be very close to the point of sitting down and eating. Indian restaurants don't bring out your (already cooled, why not?) flatbread as they seat you and let it cool even more, or bring it straight from a refrigerator. It's FRESH as in basically just done in time for serving the meal, at least around here. Have a dipping fluid, even if providing modern gravy as well, and even if not butter. Think about pushing folks to tear some off, dip it in the dipping fluid, to personal extent, then using fork and spoon to add meat slices (not chunks) and gravy to one's personal taste, and placing it in mouth using one's fingers so requiring it to not be wet, rather moist, only using fork (or... spoon... if that wet!) to get it there if one has not yet the experience to get it right. I might suggest, also, seeing how one might not include a modern gravy. Meat, for instance, that is not "well-rested" has a great deal of moisture of the "squeezed out" type than rested meat. Nicer texture too, though I digress. Yes, liquid ends up on the plate, but that'd be stage one of moistening the torn chunk of pudding, sopping up the accumulation to the moment, then using the provided dipping liquid (the butter, or an au jus), then placing meat on it and eating). NO loss of liquid or flavor that any imagined reabsorption of liquids by cells during "resting" would have saved. (Rested meat seems less moist to me, as if evaporation did more of the work to the tented foil resting meat (no tight wrapping of the meat during resting, eh?) meaning the liquid is really lost, not retained. But I've only personal experience and thinking to rely upon there.) No loss because you sop them up. Part of the fun, too.
That, I believe, would give you an experience much closer to what a person making HG's recipe would have ended up with, and how they would have physically brought it together and eaten it. If that is what you really want, then think about trying some or all of the above.
If you truly want to do as she did, and get whatever you get, that being success, then hold off until you can try this on an actual fire, a backyard kind of thing, or more likely, a modern grill which can allow for the arrangement of parts up and down to achieve what an 18th century fireplace could have and that a modern oven usually really isn't built for if it does not at least have a rotisserie feature. Practice out the physicality beforehand though so you are not making design changes with metals and liquid fats at 400 °F!!