Put the flour with sugar, vanilla sugar and custard powder in a bowl. Add the softened butter and knead everything into a smooth dough. This is very easy with the dough hook on the hand mixer.
Shape the dough into four 30 cm long rolls and chill for about 15 minutes.
Cut about 20 cookies from each roll and place on a baking sheet lined with baking paper.
Bake lightly in the oven preheated to 170 degrees for 12 minutes.
In the meantime, mix the fine sugar or powdered sugar with the vanilla sugar in a bowl and roll the hot cookies in the mixture. Let cool on a wire rack.