1. Heat oven to 350°F. Grease 9-inch square baking pan.
2. Stir together chocolate chips, marshmallows and nuts; set aside. Place butter in large microwave-safe bowl. Microwave at MEDIUM (50%) 1 to 1-1/2 minutes or until melted. Add sugar, eggs and vanilla; beat with spoon until well blended. Add flour, cocoa, baking powder and salt; stir until well blended. Spread batter in prepared pan.
3. Bake 22 minutes. Sprinkle chocolate chip mixture over top. Continue baking 5 minutes or until marshmallows have softened and puffed slightly. Cool completely in pan on wire rack. Using wet knife, cut into squares. About 20 brownies.