I've heard it's a good idea to let your batter rest overnight. Since there are eggs in it, I would think that it would have to rest in the refrigerator. I've also heard that the batter should be at room temperature when you pour it into the pan. So, should you refrigerate the batter overnight, and then let it sit outside the fridge for a three or four hours to warm it back up to room temperature?
I previously addressed this on a related question here. I cited Kenji Lopez-Alt's great article about Yorkshire puddings based on his testing a variety of recipes. For the full thing, you can find it on Serious Eats. As to resting and batter temperature, this is what he says:
This was what the tester determined made the largest difference in the quality of the finished product:
I'm going to say this: Resting your batter is the single most important step you can take to improving Yorkshire pudding and popovers. Not only do they come out taller, they also come out much tastier, with a more complex, toasty flavor. Non-rested-batter puddings taste positively flat (literally and figuratively) next to rested-batter puddings.
I'd go so far as to say that resting at least overnight is essential if you are really after the best.
- Warmer batter will create taller, crisper puddings with a more hollow core
- Colder batter will create denser puddings with a more distinct cup.
So, definitely rest your batter (in the fridge!) and pick the batter temperature that matches the outcome you'd prefer.