1. Melt the butter, mix with the sugar and add the milk. The mix should be lukewarm. If not let cool down a bit before you add the yeast. Mix in the yeast and let it rise for a couple of minutes. In a large bowl mix flour with salt. Add the yeast mix and egg yolks and mix. Knead for about 8-10 minutes until you get a smooth and soft dough. Cover with a kitchen cloth and let rise for 1-2 hours until the volume has doubled in size.

2. In the meantime prepare the streusel and filling. For the streusel mix flour, sugar, butter, cinnamon and walnuts (optional) until you get nice streusel. Store it in the fridge until you need it again. For the filling melt half of the chocolate and mix with the butter until well combined. Set aside. Chop the remaining chocolate coarsely. Crumble the marzipan (optional). Set all aside. Right before the dough is done rising, beat the egg whites until stiff peaks form. Add sugar and cinnamon and mix.

3. Knead the risen dough again shortly. Cut in half (if you make two loaves). On a floured surface roll out the dough to a rectangle with 30x45cm. If you prepare two loaves, divide the toppings for the two loaves and prepare one after another. Start with the melted chocolate. Spread it on the dough and leave a 2cm border on the sides. Spread the beaten egg whites on top, and sprinkle with chopped chocolate and marzipan. Roll up the dough from the long side to a firm roll. Twist the roll once or twice and connect the ends. Twist the ring again once or twice and place in a greased loaf pan (at least 12 inches long if you make only one loaf). Let it rise again for 30 minutes.

4. Preheat the oven to 180°C (350°F). Mix heavy cream with the egg yolk and brush the babka(s) with it. Sprinkle the streusel on top and bake for about 55 minutes. Check after 35 minutes. To avoid the top getting too dark, cover with aluminum foil and continue baking. After the 55 minutes reduce the heat to 165°C (330°F) and bake for another 12 minutes. The babka is ready when it sounds hollow when knocking on the bottom of the loaf. Take it out of the oven and leave it in the pan for 15 minutes. Remove from the pan and let cool on a cooling rack.