Position a rack in the middle of the oven. Heat the oven to 300°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 minutes. 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 minutes. 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.
If you like, serve the shortbread topped with Lemon Curd.
nutrition information (per serving):
Photo: Rita Maas