While it's impossible for me to know exactly what happened in any specific occasion without my actually being there (Not even I'm not THAT awesome ;), these are the top 5 things you should look out for to keep your cake from sinking the next time you bake:
1. Old Baking Powder -- Baking powder may only account for a tiny percentage of your entire cake ingredients, but it can ruin the whole thing if you're not careful! Remember that baking powder only stays fresh for about 6 months to a year, so date them when you buy them, and toss and replace any containers that have been hanging around too long.
Not sure if yours is still good? Take 5 seconds to test it before you start baking by placing a teaspoon of baking powder in about a 1/2 cup of hot water. If still good, it should start to bubble rapidly. If nothing (or barely nothing) happens, it's time to head to the store.
2. Too Much Leavening -- As counter-intuitive as it might sound, adding too much baking powder, baking soda, or yeast to a cake will cause it to sink as the amount of air that is created within the cake will be more than the structure can support and the whole thing will come crashing down.
Never add additional baking powder or other leaveners to self-raising flour or cake mixes (they already have it mixed in), and always be sure to read a recipe clearly and measure carefully.
When in doubt, remember that the average ratio for baking powder to flour is 1 to 1.5 teaspoons per cup of AP flour; so if you read a recipe that calls for something way above that, it's probably an error.
3. Overbeating -- this is probably one of the most common reasons why cakes sink. I'm not sure what it is, but we all seem to have a natural tendency towards overbeating cake batter until it is smooth and creamy. This is even easier to do when we rely on the trusty old Kitchen Aid or food processor to do our mixing for us. But beating in too much air into the batter once the dry and wet ingredients are combined will only cause the batter sink.
Go ahead and work the air in when creaming the butter, sugar, and eggs, but as soon as you add the flour mixture, remember that it's ALL about the light hand. Fold the dry ingredients through the wet only until they are just combined, then delicately divide and pour into your cake pans. If adding anything at the end (food coloring, chocolate chips, nuts, etc.), continue to work the addition through the batter as gently as possible in a flowing folding motion.
4. Oven Temperature -- an oven that isn't properly calibrated and runs either too hot or too cold, could easily make for a falling cake. If possible, spring for an external oven thermometer (like this one) to make sure that when it says 350 on the dial, it's really 350 inside the oven.
Also, don't be tempted to peek inside that oven for at least the first 80% of the suggested baking time. Remember that each time you open the oven door, the temperature inside can drop as many as 10 degrees. These tiny fluctuations in temperature can affect the even rising of the cake.
5. Timing -- Unless a recipe specifically calls for it, don't let a finished batter sit for very long before baking. 20-25 minutes while the first batch bakes is fine; a few hours while you run out to pick up the kids and finish some errands is not.
Remember that the minute the wet and dry ingredients meet, a chemical reaction starts to take place (like those baking soda volcanoes we all made in 7th grade science class). To get a light, fluffy, and beautifully raised cake, you want that chemical reaction to take place inside the oven as the cake bakes so that the air that is created gets sealed into the baking cake. If your batter is sitting on the counter or on the fridge, the air created inside will just escape into the room, and come time for baking, there will be less to lift the cake up.
And... a few bonus tips!
- *Preheating IS important. Depending on your oven, it can take as long as 30 minutes for it to reach the optimal baking temperature. Always be sure to do that first before getting on with your recipe or you'll end up with an uneven, lumpy cake.
- *Baking Powder and Baking Soda are NOT interchangeable. Though baking powder contains baking soda, it also has other components that act as a catalyst for all that good air-creating cake-rising action, and is used in recipes that don't have acidic elements. Baking soda usually works along with an acid (lemon juice, buttermilk, yogurt, chocolate, etc.). Some recipes call for both, but that doesn't mean that you can skip one or the other; if it calls for both, be sure to use both.
- *Center your oven rack. Unless otherwise told, position your oven rack in the center and place the cake pans right in the middle of the rack. If baking two cake layers at once, place them on the same rack side-by-side; don't put one on top of the other; they won't bake evenly that way.