Heat the oven to 170C/325F/Gas 3 and lightly grease a baking sheet.

Sift the flour, ground rice and salt on to a piece of greaseproof paper. Using a wooden spoon, beat the butter in a bowl until creamy, then beat in the sugar and orange zest until thoroughly combined.

Gradually work the sifted flour mixture into the butter mixture, using a wooden spoon at first and finishing by gathering the dough into a ball with your hands.

Roll the dough out on a floured surface to a rectangle about 1 cm (1/2 inch) thick, patting the shortbread into shape with your fingers to give a neat edge. It is essential to have cool hands at this point, run them under cold water and quickly dry before handling the mixture.

With a fish slice, carefully lift the shortbread on to the greased baking sheet and prick all over with a fork. Bake in the oven for 45 minutes until lightly coloured. Immediately cut in half lengthways, then cut into fingers.

Cool for 10 minutes on the baking sheet until firm, then transfer to a wire rack to cool completely. Sprinkle with caster sugar if wished.