Heat oven to 350°F. Line 13x9-inch pan with foil so foil extends about 2 inches over sides of pan. Spray foil with cooking spray. Make brownies as directed on box. Cool completely, about 1 hour.
Using foil to lift, remove brownies from pan. With mustache-shaped cookie cutter, cut brownies into mustache shapes. Spread frosting on brownies. Top with cereal.