Preheat the oven to 350 °F. Line a 9 x 13 baking pan with parchment paper.
Mix the flour, sugar, and salt together until combined.
Add in your cubed butter and cut into the flour. You can do this by pulsing the ingredients in a food processor 4 - 5 times, using a pastry cutter, or rubbing the butter into the flour by hand.
Pour the mixture into the baking pan and press down firmly.
Bake for 25 minutes if you also need to bake the filling. If you do not need to bake the filling, continue baking until the crust is a light golden brown - about an additional 15 minutes.
Notes
You can add in extracts, zests, herbs or spices to flavor the shortbread crust. About 1 teaspoon of vanilla extract will work for this recipe and 1 - 2 teaspoons of an herb or spice, depending on its potency.
The recipe can be halved to fit an 8 x 8 or 9 x 9 baking pan. The halved recipe should also work well for most tart pans.