Place flour, sugar, vanilla sugar, and egg in the bowl of a food processor. Process until the mixture is the texture of a coarse meal.
Add the butter to the food processor. Process just until a dough forms. If the dough is too sticky, then add a spoonful of flour. If it is too dry, then add a spoonful of water.
Flatten the dough into a disc and put it in a plastic bag. Refrigerate until chilled, about 1-2 hours.
Preheat the oven to 320°F/160°C.
Line two sheet pans with parchment paper.
On a floured surface, roll out the dough to about a ½-inch (1 cm) thickness (relatively thick so that they do not break).
Cut out as many cookies as possible and place them on prepared sheet pans. *If you choose to decorate with only sprinkles or colored sugar, do it before the cookies go into the oven.
Bake for approximately 10-12 minutes, until golden at the bottom.
Let cookies cool completely, preferably on a cooling rack.
Once the cookies have cooled, you can move on to icing and decorating them!