In a bowl, beat the butter until soft, add the sugar and beat until pale and creamy. Sift in flours, work to a smooth paste. Chill 15 minutes.
Place a 15cm flan ring on a baking sheet. Press shortbread paste into a neat circle. Remove flan ring and flatten slightly with a rolling pin. Crimp edges. Prick all over with a skewer. Mark into 6-8 wedges and sprinkle with caster sugar.
Bake for 40 minutes, until a pale biscuit colour. Remove from oven and cool 2 minutes, then transfer to a wire rack to cool completely. Store in an airtight container.