Preheat the oven to 180 ° C top / bottom heat. Grease the form and dust with flour.
Separate the eggs and beat the egg whites into firm snow.
Beat the butter with the yolk until frothy. Add the lemon zest, vanilla sugar and icing sugar, stir everything together. Add the curd spoon by spoon and stir. Sieve flour with cornstarch and baking powder and stir in. Fold in the egg whites.
Pour the dough into the mold, bake in the preheated oven on the middle rack for approx. 55 minutes. Make a chopstick test.
Cool the Topfengugl in the tin, then turn it out and dust with icing sugar.