1. Preheat oven to 350°. Line bottom and sides of a 15- x 10-inch jelly-roll pan with aluminum foil, allowing 2 to 3 inches to extend over sides; lightly grease foil with cooking spray. Microwave butter and chocolate in a large microwave-safe bowl at HIGH 1 1/2 to 2 minutes or just until melted and smooth, stirring every 30 seconds. Whisk in sugar. Add eggs, 1 at a time, whisking just until blended after each addition. Whisk in flour and next 3 ingredients. Pour mixture into prepared pan.
2. Bake at 350° for 18 to 20 minutes or until a wooden pick inserted in center comes out with a few moist crumbs. Cool completely on a wire rack. Lift brownies from pan, using foil as handles. Remove foil; cut into 24 squares.