Melt the chocolate in a water bath and let it cool down a bit.
In the meantime, mix butter, sugar, vanilla sugar and salt with a hand mixer. Add four egg yolks and stir in. Gradually mix in flour and knead everything into a smooth dough. Divide the dough in half and knead one half with the melted chocolate. Wrap each of the two types of dough in plastic wrap and place in the refrigerator for an hour.
Towards the end of the resting time, mix egg whites with a dash of milk.
Roll out the doughs on floured work surface to 1 cm thickness, cut into 1 cm wide strips and brush them with the egg white milk. Then place the strips on top of each other in a checkerboard pattern and press down lightly. Wrap the resulting dough blocks in plastic wrap and place in the refrigerator for another hour.
Line a baking sheet with baking paper and preheat the oven to 180 °C. Unwrap the dough blocks from the foil and cut them into 5 mm thin slices. Place the cookies on the prepared tray and bake in the hot oven for about 12 minutes.