1. Preheat the oven to 350°F (175°C). Line an 8- or 9-inch square baking pan with parchment paper, leaving an overhang on two sides, and lightly grease the parchment and any exposed pan edges.

2. In a medium microwave-safe bowl, melt the unsalted butter in short bursts until just liquefied, then let it cool slightly so it is warm but not piping hot.

3. Whisk the granulated sugar into the melted butter until the mixture looks thick and glossy, then add the eggs and vanilla extract and whisk until the mixture is smooth and slightly lighter in color.

4. Sprinkle the cocoa powder, all-purpose flour, baking powder, and fine sea salt over the wet ingredients, then switch to a spatula and gently fold just until no dry streaks remain. Stir in the semi-sweet chocolate chips until evenly distributed; the batter will be thick.

5. In a separate small bowl, stir together the creamy peanut butter, powdered sugar, milk or heavy cream, vanilla extract for the swirl, and a pinch of fine sea salt until smooth and spoonable. Adjust the consistency by adding a splash more milk if too thick or a bit more powdered sugar if too thin.

6. Spread the brownie batter evenly into the prepared pan, smoothing the top with a spatula. Drop spoonfuls of the peanut butter mixture all over the surface of the batter.

7. Use a butter knife or skewer to gently drag through the batter in figure-eight motions, creating a marbled swirl without over-mixing the peanut butter into the chocolate.

8. Bake for 20–25 minutes, or until the edges are set and slightly pulled away from the pan and a toothpick inserted near the center comes out with a few moist crumbs but not wet batter.

9. Place the pan on a wire rack and let the brownies cool completely in the pan so they can finish setting.

10. Once cool, use the parchment overhang to lift the brownies out of the pan, then slice into 9 large squares or 16 smaller pieces and serve.