Okay, new problem: I had found a nice butter cake recipe that I tried that came out denser than a pound cake. It calls for 3 sticks of butter, 1 cup of milk, 2 cups of sugar, 5 eggs, 3 cups of cake flour, vanilla, and only 3/4 of a teaspoon of baking powder. Guessing that there was way too little baking powder, I increased it to 3 teaspoons and found the recipe worked much betterin a cupcake. It was light and tasty. Nice crumb. When I baked the batter in one 12 inch pan, however, the outside cooked nicely while the center was all bubbly-looking and sunken. Even though the center was cooked (skewer came out clean), I definitely had a sinking problem. I baked at 350 degrees and used a flower nail in the center to act as a heating core. Im checking the accuracy of my oven tonight, but any other ideas would be appreciated!

It looks to me that there isn't enough structure to this cake to support it. If you used no more than 1 teaspoon baking powder per cup of flour, then you probably didn't over leaven it. The butter and the sugar may be too high, destroying the structure provided by the eggs enough so that the cake could not maintain it's shape. I would try lowering the amount of butter by half a stick and the sugar by a quarter of a cup; use 3 teaspoons of baking powder and see if that helps.

Too much leavening. When you "fixed" the recipe for a cupcake it worked, but for a cake the recipe might have been OK as it was at the beginning

The larger the cake, the less leavening you require.

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