Put flour, 200 g sugar, vanillin sugar, butter in flakes, salt, 1 egg and brittle in a bowl and knead with the dough hooks of the hand mixer to a crumbly mass. At the end knead further with your hands to thick crumbles. Grease a springform pan (26 cm Ø) and put half of the crumbles into the pan. Press it down as a base and sprinkle with ground hazelnuts. Peel, quarter, core and cut apples into slices. Mix apples with lemon juice and spread evenly on the base.
For the glaze, mix sour cream, 75 g sugar, 4 eggs and sauce powder and pour over the apples. Sprinkle with remaining crumbles and bake in the preheated oven, bottom shelf, (electric range: 200 °C/ convection oven: 175 °C/ gas: level 3) for approx. 50 minutes. Let the cake cool down in the tin on a cake rack. Remove the cake from the edge with a knife and take it out of the springform pan. Dust with icing sugar, cut into pieces and serve with whipped cream