For the pie dough: Add butter to a large bowl and knead with your hands until it softens. Add flour, baking powder, a pinch of salt, sugar and vanilla sugar. Knead everything into a loose dough with lumps, then add egg yolks and sour cream. Combine quickly into a firm ball of dough, cover with cling film and refrigerate for at last half an hour.
In the meantime prepare the filling: Peel apples and grate them (or cut into small pieces). Add sugar, rum, cinnamon and butter-toasted breadcrumbs, and combine thoroughly. If your apples are too juicy, squeeze out some of the moisture.
Preheat oven to 180°C. Coat pie pan with butter or oil.
Take the dough out of the fridge and divide: one cut 2/3 (for the bottom), one 1/3 (for the top). Roll out the larger dough cut into a flat circle, about 0,5 cm thick. Do that on a large towel, dusted with flour, or on cling film. Then, carefully place the dough on top of the pie pan and push the dough down and into the sides to form the crust.
Cover the dough with apple filling generously.
Take the remaining 1/3 dough, roll it out into a circle, then with a knife cut into strips about 1 cm in size. Place the dough strips on top of the apple filling in a net pattern. Rub the dough net with sourcream and then place in the preheated oven to bake for about 45 minutes.
When baked, remove from oven and let it cool completely before serving. Optionally, dust with powdered sugar.