Preheat the oven to 180 ° C. Grease a springform pan with a little butter
Mix the butter with sugar, vanilla sugar and a pinch of salt until frothy. Gradually stir in the eggs. Add the flour, custard powder and baking powder and stir well. Gradually add the milk.
Wash and peel the apples, remove the core and cut the apples into pieces as desired. Fold the apple pieces into the batter with a spatula.
Pour the batter with the apple pieces into the springform pan and bake on the middle rack for about 45 minutes