Line 13x9-inch pan with foil, with ends of foil extending over sides; spray with cooking spray. Microwave chocolate and butter in large microwaveable bowl on HIGH 2 min. or until butter is melted. Stir until chocolate is completely melted.
Add sugar; mix well. Blend in eggs and vanilla. Stir in flour and nuts; spread into prepared pan.
Bake 30 to 35 min. or until toothpick inserted in center comes out with fudgy crumbs. (Do not overbake.) Cool completely.
Use foil handles to lift brownies from pan.
Cut into heart shapes with 2-inch cookie cutter. Reserve scraps for snacking or other use. Decorate cutouts with frosting and sprinkles.
