Preheat oven to 190 C / Gas 5. Grease or line a baking tray with baking parchment.
Beat butter until fluffy. Stir in the egg, then gradually beat in sugar, hazelnuts, vanilla sugar and flour. Transfer to a piping bag with a star nozzle.
Pipe small-medium crescents or circles onto the prepared baking tray.
Bake for 10-15 minutes in preheated oven or until lightly golden.
Leave the biscuits to cool and cover them with chocolate, if desired. Allow to set and store in a biscuit tin.