HEAT oven to 350°F. Line a 13x9-inch pan with foil extending foil over sides of pan. Spray lightly with no-stick cooking spray. Prepare brownies according to package directions using oil, water and eggs. Spread into prepared pan.
BAKE 28 to 30 minutes or until set. Cool 1 hour or until completely cooled. Freeze brownies 30 to 45 minutes. Remove brownies from pan by lifting foil; remove foil. Trim outer edges of brownie; cut into 1 1/4-inch squares, brushing away loose crumbs. Place on wire rack set over wax paper; return to freezer while preparing frosting.
PLACE chocolate chips, frosting and vanilla in medium microwave-safe bowl. Microwave on HIGH 1 to 1 1/2 minutes, stirring occasionally until smooth. Remove brownies from freezer. Spoon frosting over each brownie, coating top and sides. If frosting thickens, microwave on HIGH a few seconds, stirring until desired consistency is achieved. Decorate as desired with decorator frosting.