Using either a wooden spoon or an electric mixer, cream the butter and sugar together until light and fluffy (at least 5 - 10 minutes)
Mix in the flour and the salt until completely incorporated; you may need to knead the dough together if it's looking a little crumbly (as you would pastry) but it should stick together. Add a splash of milk if it's really not working for you.
Roll the dough out thinly and cut it into shapes, rounds, or fingers. Place in the fridge for half an hour or so, until firm. Preheat the oven to 350° F.
Sprinkle the tops with sugar, chopped nuts, or cocoa nibs.
Bake for 20 minutes or so, until just golden brown. Remove from the oven and allow to stand on the baking tray for 10 minutes before transferring to a wire rack to cool.