Prepare the shortcrust pastry base with 100 g butter, 200 g flour, 1 egg, 70 g sugar and the baking powder and place in the refrigerator for about 1 hour.
In the meantime, process the remaining 150 g of flour, 80 g of sugar and 80 g of butter into fine crumble and chill as well.
Peel, core and roughly grate the apples. Mix the sour cream, 3 eggs, custard powder, 100 g sugar, vanilla sugar and walnuts into a batter.
Roll out the shortcrust pastry and place in a springform pan (26 cm), pulling up an approx. 2 cm high edge.
Spread the coarsely grated apples on the base and pour the chocolate-nut mixture over it (it is a little runny). Finally, loosely spread the fine crumble over the top.
Bake in a preheated oven at 175 degrees for about 55 - 60 minutes.