Mix the softened butter with the egg yolks, sugar and vanilla sugar until frothy. Sift the flour over it and work everything into a firm dough. Let the dough rest in the refrigerator for 30 minutes.
For the filling: peel, core and slice the apples. Mix the apple slices with sugar, ground nuts, cinnamon and lemon peel spice. Halve the dough. Roll out both halves of the dough between cling film to form a round plate. Line the bottom of a springform pan (26 cm) with baking paper, grease the edge. Place the first sheet of dough in the springform pan, push up a little on the edge. Add apple filling. Place the second sheet of dough over it and press it down a little on the edge. Prick 2-3 times with a fork.
Whisk the egg white and brush the flatbread with it. Put the flatbread in the oven at 180 ° C c. Bake until golden yellow for 35 - 40 minutes. Serve warm or cold dusted with powdered sugar.
Variation: You can also use sour cherries instead of apples. For the filling, core 750 g of sour cherries, mix with 3 tablespoons of ground nuts, 100 g of sugar and 3 tablespoons of kirsch.