Mix butter, 1 egg yolk, icing sugar and vanillin sugar. Sift the flour over it and knead it in. Form a rectangular strand (approx. 3 x 6 cm) and knead overnight.
Whisk 1 egg yolk. Spread the dough strand with it and roll in the sugar. Cut into approx. 1/2 cm thick slices. Place on 2 baking trays lined with baking paper. Bake one after the other in a preheated oven (electric: 175 °C/ convection oven: 150 °C/ gas: level 2) for about 10 minutes
Waiting time approx. 12 hours