1. Heat the oven to 190C/375F/Gas 5.
2. Beat the butter and the sugar together until pale.
3. Stir in the flour to get a smooth paste. Turn on to a work surface and gently roll out until the paste is 1cm/½in thick.
4. Cut into rounds or fingers and sprinkle with a little extra caster sugar. Put on a baking sheet and chill for 20 minutes. Bake in the oven for 15-20 minutes until pale golden.