Wash hands with soap and water
Preheat the oven to 350° degrees F. Spray a 9x9-inch square baking dish. Be sure to cover the sides with the spray as well.
In a large microwave-safe bowl, add the bittersweet chocolate and butter. Toss to combine. Microwave on high for 20 seconds and then stir. Continue to microwave in 10 to 20 second increments, stirring in between until the mixture is melted and smooth. Allow the mixture to cool for 5 to 10 minutes so you don't melt the sugar or cook the eggs when you add them.
While the chocolate cools, whisk together the flour and salt in a medium bowl and set aside. Wash hands with soap and water after handling raw flour.
Prepare the peanut butter swirl in a medium bowl by beating together the melted butter, creamy peanut butter, powdered sugar and vanilla extract. Set aside.
Once the chocolate mixture has cooled to room temperature, stir in the cocoa powder. Then, add the sugar and brown sugar. Stir well to combine.
Add the eggs and vanilla, and beat to combine. Wash hands with soap and water after handling raw eggs.
Gently stir in the flour and salt mixture along with the peanut butter chips until well combined. Wash hands with soap and water after handling raw flour.
Do not eat raw batter.
Add 2/3 of the brownie batter into the prepared pan. Use a large spoon or ice cream scoop to add the peanut butter swirl. Fill in the gaps of the peanut butter swirl with the remaining brownie batter. Use a knife to swirl the two mixtures together.
Bake at 350 °F for 30 to 35 minutes until a toothpick inserted in the center comes out clean or with a few crumbles of cooked brownie.
Cool completely before cutting. Store in an airtight container.