Put butter and sugar together in a bowl and beat until light and fluffy. Add flour and using your hands, gradually work it until the mixture forms a smooth dough. Roll into a sausage shape about 30cm long. Pat the sides to form either a square or a triangular log.
Sprinkle a thick layer of demerara sugar on a piece of baking parchment. Press each side of the dough into the sugar to coat it.
Wrap in cling film and chill in the fridge for 30 minutes until firm.
When ready to bake, preheat oven to Gas 3. Remove dough from cling film and cut into thick slices. Place slightly apart on a non-stick baking tray. Bake for 20 minutes, until they are just starting to brown.