Cut the butter into pieces. Knead the sugar, butter, flour, vanilla sugar and egg first with the dough hooks of the hand mixer, then with your hands to a smooth dough. Wrap the dough in foil and chill for about 30 minutes. Halve the dough. Knead half with cocoa.
Roll out both doughs to a rectangle (approx. 4 cm wide, approx. 2 cm thick) on a work surface dusted with flour and halve lengthwise. Put 2 light and 2 dark strands of dough on top of each other, press firmly and chill for about 30 minutes. Cut the dough into approx. 1 cm thick pieces and place on 2 baking trays lined with baking paper. Bake the trays one after the other in a preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for 8-10 minutes. Let the biscuits cool down on a cake rack
Waiting time approx. 1 hour