Put the butter and sugar in a large bowl and cream together until pale and fluffy. Add the flours 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 the pan, smoothing it with the back of a spoon. Crimp the edges by pressing with the tips of two fingers, or two knuckles, then mark into 8 pieces with a knife and prick each triangle three times with a fork. Put in the refrigerator for 30 minutes to firm up. Preheat the oven to 325 degrees F (170 degrees C).
Remove the shortbread from the refrigerator and bake in the oven for 30 to 35 minutes, or until it is a very light golden color. Remove the shortbread from the oven and sprinkle over some superfine sugar. Let cool in the pan for a few minutes before removing it from the pan and cooling on a cooling rack. Once cold, cut into 8 pieces and serve.
Variation: Add a tablespoon of grated stem ginger or the finely grated zest of 1 lemon or lime.