1. In a food processor, whirl butter, brown sugar, and flour together until smooth. Or, in a bowl, with an electric mixer on medium speed, beat butter and brown sugar until smooth; stir in flour until mixture is no longer crumbly and starts to come together in a ball. Pat dough into a 1-inch-thick rectangle, wrap airtight, and freeze until firm, about 20 minutes.
2. Unwrap dough and roll between sheets of waxed paper into a 6- by 18-inch rectangle. Cut in half crosswise, through paper. Peel off top sheet of paper and invert each rectangle onto a baking sheet. Peel off remaining paper. With a long, sharp knife, cut dough into 3/4- by 2-inch rectangles. Prick with a fork in parallel lines about 1/2 inch apart. Move rectangles about 1 inch apart.
3. Bake in a 300° oven until cookies are firm to touch and slightly darker brown, about 15 minutes; if baking more than one pan at a time, switch pan positions halfway through baking. With a wide spatula, transfer cookies to racks to cool completely.
Note: Nutritional analysis is per cookie.