Assembly

• Heat the oven to 325°F and grease an 8 inch-square baking dish. Line the pan with parchment paper so there is a 2 inch overhang on opposite sides. These will serve as handles to lift the brownies from the pan.
• Whisk together the melted butter, sugar, and cocoa powder in a large mixing bowl. Add the eggs and vanilla and vigorously whisk to combine.
• Combine the flour, salt, and baking powder in a separate bowl, then stir it into the chocolate mixture until just combined. Set aside.
• Add the butter, powdered sugar, peanut butter, and salt to a small bowl and mix until smooth.
• Use the back of a spoon or an offset spatula to spread half of the brownie batter evenly into the baking dish. (The batter will be very thick).
• Spoon half of the peanut butter swirl in tablespoon-sized dollops on top of the brownie batter.
• Top with the remaining brownie batter and spread evenly, then dollop the rest of the peanut butter filling on top.
• Insert a skewer or butter knife all the way to the bottom of the pan and drag the edge through the peanut butter dollops to make swirls.
• Bake for 30-35 minutes, until puffed and lightly browned on top. Do not overbake.
• A toothpick inserted into the center should come out a little wet with lots of crumbs.
• Cool completely, then use the parchment handles to lift them from the pan and transfer to a cutting board.
• Cut into squares and serve.