Mix the butter with sugar and gradually add flour, kneading with your hands until you can form a smooth ball that is not crumbly.
Refrigerate for an hour.
Take out the dough, sprinkle some flour on a clean surface to roll out the dough (in parts) to 1/2 cm thick.
Cut out your cookies and place on a cookie sheet.
Beat the egg (or yolk) with vanilla and brush the cookies with the mixture before baking.
Bake for 10-15 minutes, making sure to check that the cookies brown, but without burning. Use a timer!
Notes
Cookies can also be baked without egg on it, in case people have allergies. Either way, they are ready to serve, although they'll look quite boring.This can be easily fixed by making some icing with powdered sugar and lemon juice, berry juice, or any other liquid (with/without color). You will only need a few drops so the cookies can be painted with a brush.Other decorating options are to add raisins or nuts before baking.