Warm up the milk. Melt butter in it. Mix 400 g flour, yeast, sugar and 1 pinch of salt. Add egg and egg yolk. Add warm milk-butter mixture. Knead the dough first with the dough hooks of the mixer, then with your hands for 5-6 minutes to a smooth dough.
Cut the dough in half. Knead cocoa under one half of the dough. Form both halves of dough into a ball and put them back into the bowl. Cover and leave to rise in a warm place for about 1 1⁄2 hours. Knead the dough again with your hands. Then let it rest in the fridge for at least 3 hours.
Knead both doughs briefly. Roll out one after the other onto some flour to a thickness of about 1 cm. Place the rolled out light and dark doughs on top of each other and roll up to a roll. Then roll out to a thickness of approx. 1 cm. This way the layer pattern is formed. Cut out 13-15 donuts (each approx. 8 cm Ø) with a donut cutter or glass rim, but do not remove the middle piece.
Knead dough remains again and roll out. This creates a marble pattern. Then cut out again. Cover the filled dough circles and let them rise. Fry in portions. Lift out, drip off on kitchen paper and let cool down.
Remove enough of the dough from the inner circle that a firm base remains. Fill the donut bowl with custard. For the icing, stir 50 g icing sugar and lemon juice until smooth. Dribble in strips over the donuts. Leave to dry. Dust with some icing sugar.