01 - Preheat the oven to 350°F (175°C) and grease a square baking pan.
02 - In a large bowl, whisk together melted unsalted butter, granulated sugar, eggs, and vanilla extract until smooth.
03 - Stir in cocoa powder, all-purpose flour, salt, and baking powder until just combined; do not overmix.
04 - Pour half of the batter into the prepared pan. Drop spoonfuls of creamy peanut butter over the batter, then spread the remaining batter on top. Use a knife to gently swirl the peanut butter through the batter to create a marbled effect.
05 - Bake for 25–30 minutes or until the brownies are set and a toothpick inserted in the center comes out with a few moist crumbs.
06 - Allow brownies to cool in the pan before slicing and serving.