Mix butter, sugar, vanilla sugar and eggs to a foam mixture. Then rub the lemon peel, add the juice of half a lemon and mix in the oat milk. Alternatively, of course, any plant-based drink can also be used, but also milk. When it came to specifying fluids, I went very much by feeling. Just add enough to get the consistency of a batter. Then stir in the flour mixed with baking powder and protein powder.
Then stir in the apple slices. However, you should leave about 1 apple for the surface. Put the dough with apple pieces in a greased springform pan and drape the remaining apple slices in a circle on the surface.