For the plain cookies, cream the butter with the sugar and vanilla sugar for 5 minutes. Slowly stir in the egg yolks. Mix flour and baking powder and slowly fold into the dough.
Line a baking tray with baking paper. Using a small spoon, scoop out balls of the mixture and place on the baking sheet.
Bake the cookies in a preheated oven at 180 degrees for about 10 minutes until golden brown.