1Heat oven to 350°F (325°F for dark or nonstick pan). Line 8- or 9-inch square pan with foil so foil extends about 2 inches over sides of pan. Spray foil with cooking spray. Make brownies as directed on box for 8- or 9-inch square pan. Cool completely, about 1 1/2 hours. Remove from pan by lifting foil; peel foil away. For brownies, cut into 4 rows by 4 rows.
2Heat frosting in microwavable bowl uncovered on High 30 seconds, stirring every 10 seconds, until frosting can be stirred smooth and fluid. If frosting becomes too firm while decorating, microwave 5 seconds; stir.
3Top each brownie with 1 large marshmallow. Spoon 1 tablespoon frosting over each marshmallow to coat. Let stand until frosting is set, about 30 minutes. Use black gel to make eyes and mouths.