1. Preheat oven to 350°F (175°C). Line an 8×8 inch baking pan with parchment paper, leaving some overhang, and lightly grease.

2. In a medium saucepan, melt the unsalted butter and chopped semi sweet chocolate over low heat, stirring until smooth. Remove from heat and let cool slightly.

3. Whisk the granulated sugar and brown sugar into the warm chocolate mixture until combined and glossy.

4. Whisk in the eggs one at a time, then whisk in the vanilla extract until the batter is thick and smooth.

5. Using a spatula, fold in the flour, cocoa powder, and fine sea salt just until no dry streaks remain. Fold in the chocolate chips if using.

6. Spread about two thirds of the brownie batter into the prepared pan and smooth the top.

7. In a small bowl, stir together the creamy peanut butter, powdered sugar, milk or heavy cream, vanilla extract for the swirl, and pinch of salt until smooth and slightly pourable.

8. Dollop spoonfuls of the peanut butter mixture and the remaining brownie batter over the base layer, then drag a knife or skewer through the pan to create a swirl pattern.

9. Bake for 32 to 38 minutes, or until the edges are set and a toothpick inserted near the center comes out with a few moist crumbs.

10. Cool completely in the pan on a wire rack, then lift out using the parchment overhang and cut into 16 squares.