Make the dough by combining flour, butter, and sour cream. Let it rest in the refrigerator for at least half an hour. Meanwhile, prepare the filling by mixing ground walnuts with powdered sugar, lemon zest, and vanilla sugar. Pour warm milk over it and create a mixture. Roll out the rested dough into a rectangle and cut it into equal squares. Fill with the walnut mixture and join two opposite corners of each square. Transfer to a baking sheet lined with parchment paper and bake at 200°C for about 10 to 15 minutes. Once done, dust with powdered sugar.