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.
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.
Preheat oven to 32
Place dough on a baking sheet coated with cooking spray; press dough into an 8 x 5inch 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.
