Prepare a dough from margarine+ sugar+ eggs+ flour+ baking powder and bake in the springform pan for 12 - 15 minutes at 200 °C.
Heat up applesauce and 1 tablespoon of sugar, bring to the boil together with vanilla custard powder (mixed with a little milk), stir vigorously. Pour the applesauce mixture onto the pre-baked base.
Stir the sour cream+ sugar+ egg yolk, except for the egg whites, which are stirred in until stiff. Pour the mixture onto the applesauce and bake again at 200°C for about 20 minutes.