1cupquality unsalted butter , cubed and softened at room temperature (the better the butter, the better the shortbread)
1/2cupcaster sugar, or "baker's sugar" in the U.S. (if you can't find it simply pulse granulated sugar in a blender until very fine. Do NOT use powdered sugar)
Other options include nuts, fresh herbs, dried cranberries, etc.
Instructions
Preheat the oven to 350 degrees F. Butter a 8x8 or 9x9 inch square baking pan. You can also use a round cake can and cut the shortbread into triangles.
Place the flour, butter, caster sugar and salt in a food processor and pulse until it's combined and looks like coarse breadcrumbs but is soft and pliable and comes together in a dough when you press it together between your fingers. If it's too dry and crumbly it needs to be pulsed a bit longer. (If using any add-ins, stir them in at this point.)Pour the mixture into the greased baking pan. Use your fingers and hands to firmly press down the mixture. Note: If the mixture is too dry to work with, including pricking with a fork (see below), then it was not pulsed long enough in the food processor. Prick the shortbread with the tines of a fork, creating rows. You can also like run a knife between each row of fork tines to make cutting the shortbread easier after it's baked. Another option is to prick the shortbread with a fork immediately after it is done baking while it is still warm; this way the holes will be more pronounced as they have a tendency to close during baking.
Place the shortbread on the middle rack and bake for 30-35 minutes or until light golden and firm. Remove from the oven and sprinkle lightly with caster sugar or granulated sugar. Let cool. Cut and serve.Store the shortbread in an airtight container in a cool place (not the fridge) for up to several weeks. Its flavor and texture improves over time.