Finely grate the chocolate. Mix flour and baking powder in a bowl and make a depression in the middle. Add eggs, 125 g sugar, vanillin sugar, speculoos spice and salt to the hollow and mix with some flour from the rim to a thick paste. Cut 125 g butter into cubes and add to the mash with quark. Knead first with the dough hooks of the hand mixer, then with your hands to a smooth dough
Remove approx. 1/3 of the dough, knead with 1 tbsp. milk, cocoa and grated chocolate. Roll out the light-coloured dough into a rectangle (approx. 32 x 37 cm) on a floured work surface. Spread with 2 tablespoons of milk. Roll out the chocolate dough on a floured work surface in the same size. Place on the light-coloured dough rectangle. Roll out 1/3 of the dough from one long side. Roll up from the other side as well, overlapping the larger roll slightly on the small roll, press together slightly. Lay diagonally on a baking tray lined with baking paper
Melt 100 g butter. Spread the stollen with about 1/3 of the butter. Mix 50 g white sugar and brown sugar. Sprinkle Stollen with approx. 1/3 of the sugar mixture. Bake in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: see manufacturer) for 40-45 minutes. After approx. 30 minutes, brush the stollen with 1/3 of the butter and sprinkle with 1/3 of the sugar. Remove the stollen from the oven, brush with remaining butter and sprinkle with remaining sugar. Let them cool down. Serve with butter to taste