Cream the butter and sugar together in a large bowl until pale and fluffy. Add the plain flour, rice flour and salt, and stir well until the mixture is smooth and uniform. (I find it easiest to keep pressing the mixture against the side of the bowl to mix it.)
Bring the mixture together and press into a 20cm/8in loose-bottomed cake tin, smoothing it with the back of a spoon. Crimp the edges by pressing with the tips of two fingers, then score into eight pieces with a knife and prick each triangle three times with a fork. Put in the fridge for 30 minutes to firm up.
Preheat the oven to 170C/325F/Gas 3.
Remove the shortbread from the fridge and bake in the oven for 30–35 minutes, or until pale golden-brown.
Remove the shortbread from the oven and sprinkle over some caster sugar. Leave to cool in the tin for a few minutes before removing it from the tin and cooling on a wire rack.
Add one tablespoon of chopped stem ginger or the finely grated zest of a lemon or lime to vary the recipe.