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 1-1/2 minutes or until melted.
Add sugar, vanilla and eggs; beat with spoon until well blended.
Add flour, cocoa, baking powder and salt; stir until well blended.
Spread batter into 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 in pan on wire rack.
Using wet knife, cut into squares.