Cut the cold butter into small pieces and rub into the flour until it resembles breadcrumbs. Add icing sugar, hazelnuts and the egg.
To cool your hands plunge into cold water and quickly knead mixture into a dough. Chill for half an hour.
Take a portion of dough and roll into sausages about 3cm in diametre then cut into 1cm wide discs. Then roll this into another sausage shape and form into a crescent. Place on lined biscuit tray and continue with the rest of the dough.
Bake for 10-15 minutes, or until very light gold in colour.
While they are baking mix the icing sugar and the vanilla sugar. Soon as the biscuits come out of the oven roll them in the icing sugar mix. Cool on rack.