Preheat the oven to 160C/325F/Gas Mark 3.
Sift the dry ingredientsinto a large bowl or food processor. Add the residue of bran from the sieve. Then put in the butter or margarine and the sugar. Whizz, or beat, all the ingredients together until they form a soft dough which leaves the sides of the bowl clean. Grease a 19 cm x 29 cm shallow tin with butter.
Press the dough into the tin, leveling the surface by pressing with the back of a metal spoon. Prick the surface all over with a fork. Bake for about 45 minutes, until the shortbread is set, quite crispy on top and very lightly tinged with gold. It's a little more tricky to tell when these are done than with normal shortbreads because this dough is already quite brown. If you have a doubt, you can lift up one of the end pieces and look underneath it to see if it looks done. If not, carefully put it back and cook for a bit longer. When it's done, cut it into sections and leave it to cool and crisp up in the tin.
This shortbread keeps well in a tin for several days and also freezes well. It defrosts very quickly; you can use it almost straight from the freezer.
