To prevent mini cheesecakes from cracking, do not beat in the egg with an electric mixer. This inflates air into the batter, which sinks when the cheesecakes come out of the oven. Instead, beat the egg very well in a separate bowl, and then stir it into the cheesecake batter. Also, as a preventative step, let the cheesecakes cool in the pan near the oven. Drastic temperature changes can crack cheesecakes, so never place your freshly baked hot cheesecakes into the freezer for faster cooler. Let them cool to room temperature gradually, and then refrigerate.