When making a coca-cola cake I follow the directions exactly. But when it comes out of the oven it appears all the miniature marshmellows went to the center and sank. Hard to frost when the middle is sunken in. What am I doing wrong?
Without looking at the recipe, I can't know for sure, but I have a couple of ideas. First idea is to check your oven temperature. Oftentimes ovens will run hot or cold, and you might not know it for years. Get an oven thermometer and match the pre-heat temperature to the temperature on the thermometer. That will help your cakes bake more exactly.
Next idea. You may be putting too much leavening in the cake. Too much baking soda or baking powder could cause your cake to fall, and it also can impact the flavor.
It may be something about the recipe itself but often cakes sink in the middle because they rise too quickly in the over and then when the cake cools there isn't enough structure to support the middle. You can alleviate this problem fairly well by dropping the temperature you bake the cake at by about 25 degrees or so and then adding 10-15 minutes to the bake time.
It might be the type of pan, is it a darker colored pan? The batter is probably heating more around the outside, so as it sets, it's pushing the marshmallows toward the middle (either that or it happens when you pour it, which I doubt). They sell A wrap for cake pans that you soak in water first. It's similar to using a water bath in that it holds some liquid on the edge of the pan to keep it from getting too hot too quickly. Or you could try a waterbath and see if that makes a difference.