Heat oven to 350°F. Grease 9-inch square baking pan.
Stir together chocolate chips, marshmallows and nuts; set aside. Place butter in large microwave-safe bowl.
Microwave at HIGH (100%) 1 to 1½ minutes 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.