Preheat the oven to 350°F. Line a 9- x 9-inch baking pan with parchment paper, leaving an overhang on two sides. Set aside.
In the bowl of a stand mixer (or just a large bowl if using a hand mixer), beat the butter and sugar until smooth.
Add the flour (and salt, if needed), and mix until a smooth dough forms.
Pat the dough into the prepared baking pan. Using plastic wrap and a pie pan roller helps get everything even without making too much of a mess. Pierce the dough with a fork, then chill for 20 to 30 minutes, or until the dough is firm.
Bake the shortbread for 25 to 30 minutes or until lightly browned. Remove from the oven to a wire rack.
Cut the shortbread into 1- x 2-inch rectangles while it's still warm in the pan. Cool the pan on a wire rack for 15 minutes.
Carefully remove the shortbread from the pan using the parchment paper overhang to the wire rack. Cool completely & enjoy!
This recipe can easily be doubled, just bake in a 9- x 13-inch baking pan for 30 to 35 minutes.You can use either unsalted or salted butter for shortbread. If you do use unsalted butter, add ½ teaspoon of salt in with the flour.