In a bowl, beat the softened butter with sugar and egg yolks. Add flour with vanilla sugar and baking powder. Add ground poppy seeds, lemon zest, milk and mix thoroughly. Finally, lightly mix in the foam from the egg whites.
Preheat the oven to 360° F. Grease the Bundt cake form with butter and sprinkle with flour. Transfer the dough into the form and bake for about 45 minutes.