Cream the butter and sugar together until pale and fluffy.
Work the sifted the flour and rice flour into the creamed mixture until it forms a firm paste.
Knead it lightly on a well-floured board.
Roll it out until quite thin (0.5 cm thick) and cut biscuit shapes with cutter of your choice.
Using a palette knife, lift the biscuits onto a well-buttered baking sheet and prick with fork.
Bake on the centre shelf of a moderate oven, 190C/375F/Gas 5, until pale golden in colour.
Remove from oven and sprinkle liberally with caster sugar while still warm. Lift on to wire tray to cool and firm-up. Store in an airtight tin.