Preheat oven to 300 degrees. Butter a 9 inch square baking pan, or spray it with nonstick cooking spray.

Cream butter and sugar in the large bowl of a stand mixer with the paddle attachment at medium speed until it's pale, light and fluffy.

Add the flour and cornstarch. Combine into a smooth dough, scraping down the sides of the bowl with a rubber spatula at least once.

Press dough in an even layer into the prepared pan. Prick all over with a fork, and bake 50 mins. or until golden brown.

While it’s still warm, sprinkle the whole thing with a generous dusting of white sugar. Allow the shortbread to cool slightly, then cut into fingers or squares.