HEAT oven to 350°F.
MICROWAVE unsweetened chocolate and butter in large microwaveable bowl on MEDIUM 2 min. or until butter is melted. Stir until chocolate is completely melted and mixture is well blended. Add sugar; mix well. Blend in eggs. Add flour and nuts; mix well.
POUR into 9-inch square pan sprayed with cooking spray.
BAKE 40 min. or until toothpick inserted in centre comes out with fudgy crumbs. (Do not overbake.)
HEAT broiler. Top warm brownies with marshmallows. Broil, 6 inches from heat, 1 min. or until marshmallows are golden brown. Melt semi-sweet chocolate as directed on package; drizzle over marshmallows. Cool completely.
serving size = 1 brownie (45 g)