Why is it called 'shortbread'? Well, firstly it is short, as in crumbly due to the high amount of shortening or fat, in this case butter. Think of 'shortcrust' pastry which is high in butter content and should also be crumbly. But why 'bread'? Most accounts have it that the original shortbread was indeed bread dough which was left over and then baked twice in the biscuit tradition with added sugar to make a sweetened and hard biscuit. As time went on the bread was replaced by the simply flour, sugar and butter combination we know today. This was of course more expensive and therefore reserved for special occasions such as weddings.
The rule of thumb for the quantities is 1:2:3. Or 1 part sugar to 2 parts butter to 3 parts flour. That's what I'm using here, but please adjust to your own taste. If you research shortbread recipes you'll find a plethora of different quantities and ingredients. Some call for plain flour only, others to mix in either cornflour, rice flour or semolina flour to bring an extra crunch. A good pinch of sea salt brings an extra depth. There do seem to be certain elements on which most people agree though.
Only 3 base ingredients? Use good ones. Decent caster sugar and good quality butter and flour.
As it's short, don't work the dough. It should be crumbly and melt in the mouth, overworking the dough will toughen it. Bring it together quickly.
A short resting time in the fridge before baking helps the texture.
Shortbread should be pale so a low oven and longer cooking than more conventional biscuits.
There are 3 main shapes for shortbread. The triangular petticoat tails, rectangular bars or a circular biscuit shape.