Preparation

• Heat  oven to 350°F.
• Line  13x9-inch pan with foil, with ends of foil extending over sides; spray with cooking spray. Place 16 wafers in prepared pan, trimming as necessary to fit. Break remaining wafers into small pieces; reserve trimmed and broken wafer pieces for later use.
• Break  half of 1 chocolate bar at score marks; place in large microwaveable bowl. Add butter. Microwave on HIGH 1 min. or until chocolate is almost melted. stir until chocolate is completely melted and mixture is well blended. Add sugar, cocoa powder, eggs and vanilla; mix well. Stir in flour; pour over wafers in pan.
• Bake  30 to 32 min. or until toothpick inserted in centre comes out with fudgy crumbs. (Do not overbake.) Meanwhile, chop remaining chocolate.
• Sprinkle  marshmallows and chopped chocolate over brownie; bake 3 to 5 min. or until marshmallows begin to puff. Immediately top with reserved wafer pieces; press gently into marshmallows with back of spoon. Cool. Use foil handles to lift brownie from pan before cutting to serve.