Mix the butter with the sugar and vanilla sugar until frothy and add the egg yolks. Then knead into a dough with the flour.
Chill the dough for approx. 1 hour, then roll out approx. 0.5 cm thick and cut out cookies. Brush these cookies with egg yolk and place on a baking sheet lined with baking paper. Bake at about 175 ° C for about 15 minutes.