Line 13x9-inch pan with foil, with ends of foil extending over sides; spray with cooking spray. Prepare brownie batter and bake in prepared pan as directed on package for cake-like brownies.
Sprinkle hot brownie with marshmallows.
Cook semi-sweet chocolate and milk in saucepan on low heat until chocolate is completely melted and mixture is well blended, stirring frequently; drizzle over marshmallows.
