For this Scottish variation of our Classic Shortbread, the dough is baked in a stoneware mold.
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.
Reduce speed to low. Add flour mixture all at once; mix until just combined.
Using plastic wrap, press dough into an 8-inch shortbread mold coated with cooking spray. With plastic on dough, refrigerate 20 minutes. Prick all over with a wooden skewer. Bake 45 minutes. Let cool 10 minutes. Unmold; cut into wedges. Let cool completely.