1. Weigh or lightly spoon flour into a dry measuring cup; level with a knife. Combine flour, cornstarch, and salt in a small bowl; stir with a whisk.
2. Place brown sugar and butter in a medium bowl; mix with hands until combined. Add flour mixture, and mix with hands until combined. Sprinkle dough with 1 1/2 teaspoons ice water; knead dough lightly 4 times or just until smooth. Wrap in plastic wrap; refrigerate 30 minutes.
3. Preheat oven to 325°.
4. Place dough on a baking sheet coated with cooking spray; press dough into an 8 x 5–inch rectangle about 3/8-inch thick. Pierce entire surface liberally with a fork. Bake at 325° for 25 minutes or just until set and edges are golden. Cut shortbread into 24 pieces. Cool completely.