Heat oven to 350°F.Grease 9-inch square baking pan.
Stir together chocoloate chips,marshmallows and nuts; set aside.Place butter in large microwave-safe bowl.Microwave at HIGH (100%) 1 to 1½minutes or until melted.Add sugar,vanilla and eggs, beating with spoon until well blended.Add flour, cocoa, baking powder and salt;blend well.Spread batter in prepared pan.
Bake 22 minutes.Sprinkle chocolate chip mixture over top.Continue baking 5 minutes or until marshmallows have softened and puffed slightly. Cool completely. With wet knife, cut into squares.