Wash the orange, rub dry and remove the zest with a zest ripper. Sieve the icing sugar into a bowl. Add flour, salt, vanillin sugar and butter in small cubes and knead everything first with the dough hook of the hand mixer, then with your hands to a smooth dough. Halve the dough. Knead under one half cocoa and under the other orange zest. Wrap both doughs in foil and chill for about 30 minutes
Halve both halves of the dough again. Roll out one half each of light and dark dough separately between 2 layers of baking paper dusted with some flour to a dough sheet (approx. 16 x 25 cm; 2-3 mm thick). Form 4 rolls (approx. 7-8 mm thick; 25 cm long) from the other half.
Spread the dough sheets and rolls with egg white. Place 2 light and 2 dark rolls each in a checkerboard pattern on the dough sheets and roll them up carefully. Wrap in foil and press the rolls into a rectangular shape with your hands and chill for approx. 30 minutes
Cut dough rolls straight at the edge. Cut rolls into approx. 0.5 cm thick slices and spread them on 3 baking trays lined with baking paper. Bake one after the other in a preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for 10-12 minutes. Let the cookies cool down a little on the baking tray first, then let them cool down completely on a cake rack
Waiting time approx. 1 hour