One thing that distinguishes the several ‘rules’ about making shortbread that have been handed down is their conflicting advice.
Thus one old Scottish cookbook of mine suggests that it should be ‘browned to taste’: a more modern one recommends no colouring at all on the surface. There is a consensus that butter – and only butter – should be used: yet I have a friend who makes hers with margarine and I have to admit that it tastes very good.
For my part, I would say that shortbread mixture should not be over-cooked: it ought to be a pale golden colour and not golden brown. And I certainly think that the mixture shouldn’t be over-worked, because the heat in your hands can turn the fat oily.
To overcome this the method I now use involves beating the fat with a wooden spoon to soften it, then gradually working it into the other ingredients. This way it is only handled at the end. When it is mixed, you can then either just pat and spread the dough straight into a prepared tin, or else roll out the dough and cut the biscuits out.
Some say the latter method makes the shortbread tough because it involves extra handling, but I’ve not found this.