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1
Preheat oven to 325; butter a 10x6 inch glass baking dish.
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2
Add butter to a mixing bowlthe butter must be very soft, the consistency of mayonnaise or whipped cream.
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3
Add the salt to the butter and mix well with a wooden spoon or whisk sot that it dissolves completely before you add the rest of the ingredients.
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4
Sift the flour and cornstarch together into a bowl.
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5
Add the sugar to the butter and mix just until combined.
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6
Add the flour mixture and mix just until a smooth dough forms.
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7
Pat the dough evenly into the prepared baking dish; the dough should be no more than 2/3 inch deep.
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8
Bake until the top and bottom are lightly browned, about 30 minutes.
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9
The middle of the shortbread should remain light; let cool on a wire rack until warm to touch.
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10
Sprinkle the shortbread with the superfine sugar; tilt the dish so that the sugar fully and evenly coats the surface and then tip out the excess sugar.
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11
With a very thin, sharp knife, cut the shortbread into rectangular fingers, about 1/2 inch wide and 2 inches long; if the cookies have become cold, they will not slice well, so they must still be warm to the touch at this point.
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12
Chill thoroughly before removing from the baking dish; the first cookie is difficult to remove, but the rest should come out easily with the aid of a small, thin offset spatula.
