Other options include nuts, fresh herbs, dried cranberries, etc.
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 caster sugar, flour, salt and butter 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. Optional: Prick the shortbread with the tines of a fork, creating rows. Some people also like run a knife between each row of fork tines to make cutting the shortbread easier after it's baked. You can also prick the shortbread with a fork immediately after it is done baking while it is still warm; the holes will be more pronounced this way 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. Let cool. Cut and serve.Store the shortbread in an airtight container for up to several weeks. Its flavor and texture improves over time.