Cream butter in pieces, 1 egg yolk, icing sugar, vanilla sugar and 2-3 tablespoons of ice-cold water with the whisks of the mixer. Sift the flour over the mixture and knead.
Form the dough into two rolls (approx. 5 cm Ø). Wrap in foil. Leave to rest in the refrigerator for at least 3 hours, preferably overnight.
Line two baking trays with baking paper. Preheat oven (electric cooker: 175 °C/circulating air: 150 °C/gas: see manufacturer). Whisk 1 egg yolk. Brush the dough rolls with it all around and roll in the decorating sugar.
Cut into approx. 1/2 cm thick slices and distribute on the sheets. Bake in the oven one after the other for about 10 minutes. Leave to cool on the trays. Store cool and dry in cookie tins.