1Heat oven to 350°F. Spray bottom only of 13x9-inch pan with cooking spray. Make and bake brownies as directed on box for 13x9-inch pan, using water, oil and eggs. Cool completely, about 1 hour.
2For brownies, cut into 6 rows by 4 rows.
3To make marshmallow flower garnish, cut 4 to 5 slits halfway through each marshmallow with scissors; open slits to form flower. Attach 1 flower to top of each brownie with small amount of frosting. From green marshmallows, cut small pieces for leaves; attach around flowers with frosting.