Preheat oven to 350 degrees. Line an 8 inch square metal pan with non-stick foil or line with regular foil and grease the foil.
In a mixing bowl, using medium speed and high speed of an electric mixer, beat butter and brown sugar until soft. Beat in vanilla and peanut butter, then add eggs and beat just until mixed in. Scrape sides of bowl and beat in baking powder and salt, making sure they are evenly distributed; Add flour and stir until it is mixed in. Spread peanut butter batter in pan.
Make swirl mixture. Melt chocolate and butter in a small microwave-safe bowl at 50% power. Alternatively, you may use a double boiler or bowl set over a pot of barely simmer water. Remove melted chocolate from heat and stir in Nutella. Drop spoonfuls of Nutella mixture over peanut butter brownie batter and drag a knife through to make a marbled pattern.
Bake on center rack for 30 minutes or until brownies begin to pull away from the side of the pan and center seems set. Cool pan on a wire rack. When brownies hit room temperature, chill them for an hour or so. Lift from pan and cut into squares.