Adjust oven rack to the middle position and heat oven to 350 degrees F. Lightly spray pan with nonstick cooking spray, line 9x13-inch baking pan with 2 pieces of foil, leaving a 3-inch overhang on all sides. Spray with nonstick cooking spray again, and set aside.

In a large heat-proof bowl add chopped chocolate and butter. Microwave on 50% power for 1 minute. Stir, and microwave on 50% power for 30 seconds. Stir, and repeat until chocolate and butter are melted and smooth. Whisk in cocoa powder until smooth. Set mixture aside to cool for 5 minutes.

Whisk eggs, sugar, vanilla, and salt into chocolate mixture until combined; then mix in flour and baking powder with a wooden spoon until combined.

Pour batter into prepared pan, smooth into corners, and level surface. Evenly place dollops of peanut butter on top of the brownie batter. Using a butter knife create swirls by running the butter knife through the batter. Make sure you don’t touch the bottom layer of foil with the knife.

Bake on middle rack until slightly puffed and a toothpick inserted in center comes out with just a few moist crumbs, about 25-30 minutes.

Cool brownies to room temperature on wire rack, about 2 hours. Remove brownies from pan using foil overhang, and cut into 16-24 squares.