For the dough, stir together the butter, sugar, vanilla sugar and lemon zest until frothy. Mix in the eggs one by one. Mix the flour with the baking powder and cornstarch and sieve, gradually stirring into the dough.
Pour the dough into a greased and floured 26-inch springform pan and smooth it out.
Peel and core the apples and cut into very thin wedges. Distribute evenly on the dough and sprinkle with a little sugar and cinnamon if you like.
Sort the blueberries, wash, dry and pour over the apples.
Process flour, sugar, vanilla sugar, almonds, cinnamon and cold butter flakes into sprinkles and distribute evenly over the cake.
Bake in the preheated oven at approx. 180 ° C for about 40 to 50 minutes until the crumble is nice and golden brown. If the crumble browns too quickly, cover it with aluminum foil. Let the cake cool a little after baking, then remove it from the mold.
Sprinkle with powdered sugar if you like before serving.
A dollop of whipped cream goes very well with this.