Line 15x10x1-inch pan with waxed paper; spray paper with cooking spray. Fill 3-quart saucepan half full with water; heat to simmering. In 2-quart heatproof bowl, place chocolate chips; place bowl over pan of simmering water. Cook 4 to 5 minutes or until melted and chips can be stirred smooth. Remove from heat. Stir in marshmallows just until blended.