Apple Sheet Cake with Crunchy Crumble Topping

In a large bowl, beat the eggs, sugar and vanilla sugar together. Add 100 g of softened butter, the sour cream and mix well. Then add 220 g of flour, baking powder and a pinch of salt, and mix everything until evenly combined. Peel the apples, cut them into slices and place them in a bowl. Add the lemon juice, 5 g of cinnamon, 20 g of brown sugar and the potato starch, and mix well. In another large bowl, combine 100 g flour, 60 g of brown sugar, 1 g cinnamon and a pinch of salt. Add 70 g of cold butter, cut into pieces, and rub the mixture together with your hands until it forms crumbs. Line a baking tray with baking paper. Pour in the prepared cake base dough and spread it out evenly. Arrange the apple slices on top, then generously cover them with the prepared crumble mixture. Bake in the oven at 180°C for 40–50 minutes. Decorate by dusting with powdered sugar. Enjoy!