Step 1Heat the oven to 300 degrees.
Step 2Cream the butter and sugar in the bowl of an electric mixer. Sift together the sugar, cornstarch, flour and salt. Add the dry ingredients a little at a time and blend to get a soft but manageable dough.
Step 3Roll the dough out on a lightly floured surface to 1/3 inch thick. If the dough is too dry to roll out easily, add a tablespoon of water to the dough. Using a 3-by-1-inch cutter or other shapes, cut the dough, then transfer the shapes to a baking sheet, placing them about 2 inches apart. Prick the tops with a fork.
Step 4Bake the shortbreads until just firm and very pale golden, about 30 minutes. Remove from the oven, let cool a minute, then gently remove from the baking sheet.