Preheat oven to temperature on brownie box; line a 9x13 pan with foil and spray lightly.
Prepare brownie batter per box directions; stir in chocolate chips if using.
Spread batter evenly in pan. Dollop warmed peanut butter across top and sprinkle powdered sugar over it.
Use a knife to swirl peanut butter into batter, making marbled lines.
Bake according to box time, checking at the lower end; a toothpick with a few moist crumbs is perfect.
Cool completely in pan before lifting with foil and cutting into bars.