Thick Scottish Shortbread

Position a rack in the middle of the oven. Heat the oven to 300 degrees F. Line an 8x8-inch brownie pan with heavy aluminum foil, letting the foil extend over two sides of the pan.

Sift the flour, cornstarch, and salt into a large bowl. Add 1/2 cup of the superfine sugar and, with an electric mixer on low speed, mix to just blend the ingredients. Add the butter pieces and vanilla and mix until large (1/4- to 1/2-inch) crumbs form, about 2 min. Very gently press the dough evenly into the prepared pan. Don’t pack the dough into the pan. Bake until the top of the shortbread just begins to turn golden, about 1 hour and 10 min. Remove from the oven and immediately sprinkle the remaining 2 Tbs. superfine sugar over the top. Cut the shortbread into 12 squares, being sure to cut through to the bottom. Cool completely before lifting the foil and shortbread from the pan.