Pre-heat the oven to 425°F.
To the mixed brownie batter, stir in the peanut butter chips. Grease an 9-inch 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 20-22 minutes. Top with the marshmallows in the last 3 minutes of baking. Remove from the oven and cut into 12 brownies.