Preheat your oven to 350°F (175°C).
Line your 9x9-inch baking pan with parchment paper.
In a medium saucepan over low heat, combine the baking chocolate and unsalted butter. Stir until melted and smooth.
In a large mixing bowl, whisk together sugar and salt. Add the melted chocolate mixture, then add eggs one at a time, mixing well after each.
Sift cocoa powder and flour into the wet ingredients. Fold gently until just combined.
In a separate bowl, mix peanut butter, melted butter, powdered sugar, and vanilla until smooth.
Pour half of the brownie batter into the pan, dollop peanut butter mixture on top, and swirl. Pour remaining brownie batter and swirl with remaining peanut butter.
Bake for 25-30 minutes, until a toothpick comes out with moist crumbs.
Cool in the pan for 15 minutes, then lift out and cool completely before slicing.
