To make the vanilla sugar, whizz the vanilla pod and sugar in a blender. Pour through a sieve and blend again. Store in an airtight jar.
Preheat the oven to Gas Mark 4, 180°C, fan 160°C. Place the flour and butter in a bowl and rub together to form crumbs. Add the vanilla sugar and mix to form a dough. Roll out the dough. Using a star-shaped cutter, cut out stars and place on a baking tray lined with nonstick baking paper.
Bake for 8-10 minutes until just golden. Sprinkle with vanilla sugar. Leave to cool. Keep for up to 1 week in a tin.