Line an 8x8 or 9x9 inch pan with parchment paper. Set aside.
Whisk together flour, sugar, and salt in a bowl.
2 cups all-purpose flour, 3/4 cup confectioners sugar, 1 1/4 teaspoons kosher salt
Place butter into the bowl of an electric mixer fitted with the paddle attachment. Beat on medium-high speed until fluffy, 3 to 5 minutes, scraping down sides of the bowl.
1 cup unsalted butter
Gradually add flour mixture; beat until pale and fluffy, about 2 minutes.
Press the dough firmly into the parchment-lined pan. Be sure the dough is evenly spread out. Place a sheet of plastic wrap on the dough, and refrigerate for 20 minutes.
Preheat oven to 300 degrees, with rack in upper third.
Remove plastic wrap. Cut dough into desired shapes with a knife. Using a toothpick or wooden skewer, prick all over at 1/4-inch intervals.
Bake until lightly golden brown and firm in the center, about 35-40 minutes. Remove pan to a wire rack. Recut shortbread into previous shapes; let cool completely.
Remove using the parchment as handles and transfer shortbread to an airtight container.