The template for shortbread is a bit like the one for marmalade, with different ingredients.
Basically, it's (by weight)
one part of caster sugar/icing sugar/brown sugar/white sugar to
two parts of butter and
three parts of flour.
The trick is in the makeup of the sugar and flour components, You can use straight flour, or a mix to taste of flour, corn flour, custard powder, with a pinch of salt added if you wish. A little wholemeal or oat bran can be used if you want a slightly coarser texture. Brown sugar gives a slightly caramelly result. Icing sugar contributes to a melting texture.
For a ploughman's shortbread, use half plain flour and half wholemeal. Include a little oatbran or oatmeal in the wholemeal component if you wish; and use brown sugar.
I experimented with Atta, a very finely ground wholemeal flour sold in Indian food stores, and priced well under conventional coarse supermarket wholemeal. As it does in a number of recipes I found the atta to produce a denser, and slightly drier product. Not a major problem when you're dunking shortbread, though. Just another option to play with.
You may find the food processor straining a little on these coarser ingredients, (120g, 240g, 360g), depending on what sort it is and how much grunt it's got, so take it easy. Maybe make a couple of smaller batches and combine before baking.
Place the ingredients in a food mixer and combine thoroughly, then tip the contents onto a bench and knead into a firm mass. (This is one recipe where the food processor demonstrates its superiority over the electric mixer.)
Form this into a cylinder of cross-section about the size you want your biscuits, and cut it into 1cm thick slices with a fine knife.
(You can gather the crumbed mix on the bench, then roll it out to 1cm thick, but it tends to be much more dense and leathery.)
Place these on a baking tray and add any fork patterns or indentations you wish. (Allow a little room between pieces as even shortbread will spread some on the baking tray.) Place in a 150C oven and bake for about 30 minutes. The bottoms should be just beginning to brown when they're done. Place on a rack to cool and store airtight.
My own personal favourite general purpose blend is to 240g room temperature butter, cut into pieces and placed in the blender, add 50-50 soft brown sugar and icing sugar (total 120g), and 360g flour component made up of 20-40g custard powder, 100-120g wheaten cornflour, and the balance 120g plain white flour and 120g wholemeal. This gives me a nutty tasting shortbread with the fragrance and fine detail of custard powder and wheaten cornflour.