Beat the butter and sugar in a mixing bowl until soft and light.
Sift in the flour and rice flour and mix to a dough.
Gently knead the dough on a lightly floured surface for a few seconds. Pat the dough out into a round about 1cm|1/2"thick. Place on a baking tray lined with non-stick baking paper. Score the top into sections and prick the top with a fork. Chill for 30 minutes.
Heat the oven to 160°C (140° fan) 325°F gas 3.
Sprinkle the top of the dough with caster sugar and bake for about 35-40 minutes, until cooked but still pale. Sprinkle with more caster sugar while still warm. Cool on the baking tray for 10 minutes, then carefully remove and place on a wire rack to cool completely.