Put the flour, sugar, vanilla sugar and the egg yolks on a clean table. Slice the butter into small cubes and add it on top. Chop the mixture with a large knife and then disperse it into a paste. Split the paste in half and add cocoa powder and cream to one of the halves. Put both parts into the fridge for 2 hours.
Now flatten the paste with a rolling pin. Cut it into stripes (50mm thick) and combine 9 black and white stripes for a checkerboard pattern, e.g.:
B W B W B W W B W or B W B B W B W B W
Place the stripes on a rolled out, rectangular piece of paste and roll it up. Wrap the roll into plastic foil and store it in the fridge for another 30 minutes. After that, cut the roll into 50mm slices and place them on a greased baking tray (or use baking paper). Preheat the oven (fan mode) to 175 degrees and bake the cookies for 12-15 minutes.
For a spiral pattern (much easier than the checkerboard), simply flatten the paste, place a black rectangular piece onto a white one and roll them up. Cool, slice and bake as above.