Preheat oven to 180°C (355°F). Place the butter and sugar in an electric mixer and beat until pale and creamy. Add the vanilla and egg yolk and beat well to combine. Add the flour and beat until a smooth dough forms. Divide the dough into 4 portions and wrap each portion in plastic wrap. Place in the refrigerator for 15 minutes or until the dough is firm.
Roll out each portion of dough between 2 sheets of non-stick baking paper until 3mm thick. Remove the baking paper and use a 7cm star-shaped cookie cutter to cut out 8 rounds from each sheet. Place the cookies on baking trays lined with baking paper, sprinkle with the clear sanding sugar and bake for 8 minutes or until the cookies are light golden. Cool on wire racks. Makes 32.
* Sanding sugar has large coarse granules. It is used to decorate cakes and biscuits.