Baking times are never exact, as there can be considerable variance in the product and environment. The three most likely variables to affect total necessary baking time are:
- How thick the biscuits were rolled
- How moist they dough was
- Actual oven temperature accuracy
For this reason, there is normally a test or indicator for doneness. The best possible test is internal temperature of the biscuit, but this recipe does not give that information, and it is difficult to get an accurate reading for something as thin as a biscuit.
The recipe indicates browning as the test, but that does not indicate the state of the interior of the biscuit, only the exterior.
Still, there are several things you can do:
- Use an oven thermometer to verify the temperature of your oven
- Roll the biscuits so that they are slightly less tall
- Be sparing with the cream, adding only as much as necessary to bring the dough together
- Of course, bake them a little longer. The trade off here is that you will get more browning on the outside the longer you bake them to get them cooked through to the center. If you are at the max time the recipe indicates already, and are getting sufficient browning, one of the other factors may be in play
Assuming your oven temperature is accurate, the most likely factor is how thick or thin the biscuits are. The cooking time will pretty much directly related to this, all other factors being equal. Even the amount of cream has less effect than you would think, because doneness is related to the internal temperature achieved (the starches will hydrolyze and taste cooked around 195°F / 90.5°C) regardless of moisture level.
I have never tried the toothpick test on a biscuit, but fundamentally, they are quick breads, much like a muffin, so it may be worth trying: stick a wooden toothpick into the biscuit to its center. When it is pulled out, there should be only minimal slightly moist crumbs sticking to the toothpick when it is done. Wet crumbs indicate under baking, perfectly clean indicates over baking.
In the end, biscuit making is a matter of experience, and you will get better at it with more batches.