Ask the chef
Why are my cakes going flat after baking? What can I do to ensure they stay fluffy?
This typically happens because people open the oven too soon. There is always a set baking time; don’t go under the baking time unless you see that it’s burning. There are many chemical and physical reactions going on in there, so when you take the heat away by opening the oven, you stop those reactions from happening, and some of those reactions can’t restart. Once they’re done, they’re done, and the cake just goes flat. One thing that makes cakes fluffy is the baking soda or baking powder component of the cake, which requires that chemical reaction. Say your baking time says 22-24 minutes. Don’t open that oven before 22 minutes. There’s a little wiggle room at the end, but if you open it at 18 minutes, it’s not even remotely close, and you’ll stunt the growth. It will just stop in its tracks, resulting in a flat or sunken cake.
— Nicole Gardner, The G-Free Spot