Preheat & Prepare Pan: Preheat oven to 175°C (350°F). Line a 9-inch square baking pan with parchment paper and grease with butter.
Mix Wet Ingredients: In a bowl, whisk melted butter, granulated sugar, and brown sugar. Add eggs and vanilla, whisking until smooth.
Combine Dry Ingredients: Sift together flour, cocoa powder, and salt. Fold into the wet mixture until just combined.
Melt Chocolate: Melt the semisweet chocolate and gently fold it into the brownie batter.
Pour & Swirl: Pour batter into the prepared pan. Drizzle warmed peanut butter on top, then use a knife or toothpick to swirl it into the batter.
Bake: Bake for 25-30 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs.
Cool & Serve: Let brownies cool in the pan for at least 10 minutes before slicing and serving.