To mixed brownie batter, stir in peanut butter chips. Grease an 8 by 13-inch baking dish with softened butter and line with foil. Spread the brownie batter into an even layer. Sprinkle the batter with chopped nuts and bake 20 to 22 minutes. Top with marshmallows in the last 3 minutes of baking. Remove from oven and cut into 12 brownies.