It’s difficult to pinpoint exactly why your cakes are sinking, since you don’t indicate whether it drops while being baked or as the cake cools. Some of the more common reasons include:
- The cakes were removed from the oven too soon and not completely baked through. This could be due to your oven running too cool, or you simply removed them from the oven before they were done.
- There was too much leavening (i.e. baking soda or baking powder) in the recipe. This would cause the mini cake to rise too quickly before the cake structure can develop. Without the cake structure, it collapses.
- The recipe has too much liquid, compared to other ingredients.
- The batter was beaten too much. Overbeating incorporates too much air. Without enough batter/cake to support this air, the structure of the cake will collapse.