Position an oven rack in the center of the oven and preheat to 325 degrees F. Butter a 9-by-12-1/2-inch jelly roll pan and line it with parchment, then butter the paper.
Whisk together the flour, cornstarch and salt in a bowl. Beat the butter and sugar with an electric mixer on medium speed until light and fluffy, 5 to 7 minutes. Slowly add the flour mixture and beat on low until the dough is just combined; do not overwork.
Transfer the dough to the prepared pan, spreading it out as evenly as possible with a spatula. Cover with plastic wrap, then use your hands to press and flatten the dough into a smooth even layer. Remove the plastic and cut the dough with a sharp knife into approximately 1-by-3-inch bars. Use a fork to prick a decorative pattern into the tops.
Bake, rotating the pan from front to back halfway through, until golden, 60 to 70 minutes. Let stand for 5 minutes in the pan, then re-cut the bars along the previous cuts. Carefully lift the bars off the parchment with a small spatula and transfer them to a wire rack to cool completely. Store the cookies in an airtight container for up to 5 days.
When measuring flour, we spoon it into a dry measuring cup and level off the excess. (Scooping directly from the bag compacts the flour, resulting in dry baked goods.)
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