Make a kneading dough from the butter, sugar, vanilla sugar, egg yolk, lemon zest and flour and let it rest in the refrigerator for 1/2 hour.
Beat the egg whites, sugar and vanilla sugar until stiff. Mix the sour cream, yoghurt and flour and then briefly stir this mixture with the egg white-sugar mixture.
Now roll out half of the kneading dough and line the bottom of a baking pan (approx. 32 x 23 cm) with it. Spread the cream on top and then cover with the second half of the rolled-out kneading dough.
Bake at 175 ° C for about 30 minutes. Allow to cool and dust with powdered sugar before serving.