Melt 125 g fat in a pot. Add 250 ml milk and remove from the heat. Mix 500 g flour, 100 g sugar, vanillin sugar, salt and yeast in a bowl. Add 1 egg. Add the milk-butter mixture and knead everything with the dough hooks of the hand mixer to a smooth dough.
Cover and leave to rise in a warm place for about 40 minutes. Roll out into a rectangle (approx. 35 x 40 cm) on a floured work surface. Place the dough on the greased, floured fat pan of the oven (32 x 39 cm).
Cover and leave to rise again in a warm place for about 15 minutes. Meanwhile roast oat flakes in a pan without fat. Knead oat flakes, 300 g flour, 150 g sugar, 375 g fat and almonds. Separate 6 eggs.
Mix poppy seed bake, egg yolk, 2 tablespoons milk and sour cream. Beat egg whites until stiff and fold in. Spread the poppy seed mixture evenly on the yeast dough. Bake in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: level 2) for 40-45 minutes.
After 20 minutes spread the crumble mixture on the poppy seed mixture. Take the cake out of the oven, place it on a cake rack and let it cool down. Decorate with caramelized oat flakes if desired.