Lemons brighten up our rich Classic Shortbread.
Preheat oven to 300 degrees, with rack in upper third. Sift flour and salt into a bowl; set aside. Put butter into the bowl of an electric mixer fitted with the paddle attachment. Mix on medium-high speed until fluffy, 3 to 5 minutes, scraping down sides of bowl. Gradually add sugar; beat until pale and fluffy, about 2 minutes. Mix in lemon zest and juice.
Reduce speed to low. Add flour mixture all at once; mix until just combined.
Halve dough, and shape into disks. Wrap each in plastic, and refrigerate 1 hour.
Working with 1 disk at a time, roll out dough on a lightly floured surface to 1/4 inch thick. Cut out rounds with a 2-inch fluted cutter. Space 1 inch apart on baking sheets lined with parchment. Refrigerate 30 minutes.
Bake until pale golden brown, 22 to 25 minutes. Transfer cookies to a wire rack; let cool completely. Dust with confectioners' sugar.