For the shorbread, preheat the oven to 170 degrees and grease a baking dish (e.g. tart pan) and sprinkle with a little flour.
Cream butter with sugar in a bowl. Add flour, rice flour and baking powder and mix everything together to form a dough. Press the dough evenly into the pan and smooth the surface.
Prick the top of the shortbread several times with a fork and bake in the oven at 170 degrees for about 40 minutes until golden brown. Sprinkle the shortbread with sugar just before serving.