Leave the butter at room temperature. If you live in a cold habitat, leave it out overnight.
Beat the butter until white and fluffy using an electric beater.
Add powdered sugar and vanilla sugar to the butter. Beat it with the slowest speed to avoid the sugar flying.
Add the egg yolk in 2 separate batches. Mix well after each time.
Add the flour to the butter mixture, and mix well. The cookie dough is basically ready. Split them into two equal halves.
If you want to add chocolate chips, mix them in the cookie dough.