Cream the butter and sugar by rubbing together. Gradually add flour and ground rice. Press into a seven-inch sandwich tin and pinch the edges to make a pattern between finger and thumb. Prick with fork and bake at 170 degrees/gas 3 for 40 minutes till pale golden. Remove from the oven and cut into eight triangles while still in the tin and leave to cool for 10 minutes. Dredge with caster sugar.