Bake brownies. Prepare batter following recipe directions. Bake and cool brownies completely. Cut into rectangles, each 2 x 3 in.
Tint icing. Divide icing and tint 1/4 cups black. Place black icing in disposable decorating bag fitted with tip 5. Place white icing in disposable decorating bag fitted with tip 10.
Decorate brownies. Use black icing and tip 5 to pipe four lines on brownies. Use white icing and tip 10 to fill in areas between black icing lines.. Use spatula to carefully swipe across brownie to blend icing. Position icing decoration.