Cream butter, 1 egg, sugar, vanilla sugar and 1 pinch of salt with the whisk of the mixer. Mix flour and baking powder, add and knead everything smooth with the dough hooks of the mixer. Halve the dough. Knead 1 half with espresso and cocoa. Wrap the doughs separately in foil and put them in a cool place for about 1 hour
Roll out doughs one after the other between two layers of cling film thinly to form a rectangle (approx. 24 x 38 cm) and cut in half lengthwise. Spread 1 light layer of dough with egg white, then place 1 dark layer of dough on top using cling film. Remove the foil, brush the surface with egg white again and place 1 light layer of dough on top. Spread again and finish with 1 dark layer of dough. Roll out the dough sheet carefully and with little pressure.
Roll up the black and white dough lengthwise to approx. 5 cm Ø. If the dough is not yet used up, cut off the rest of the dough with a knife and roll a smaller roll out of it. Halve the roll(s) of dough crosswise, wrap the pieces in foil and chill for about 2 hours.
Preheat oven (electric cooker: 180 °C/circulating air: 160 °C/gas: see manufacturer). Line two trays with baking paper. Cut dough rolls into slices of approx. 1⁄2 cm thickness. Spread the slices with a little distance between them on the trays and bake in the hot oven for 12-13 minutes one after the other. Take out and let cool down.