Preheat the oven to 350 degrees and line an 8×8" square baking pan with parchment paper or foil.

Place the chocolate and butter in a medium sauce pan and set in a pot of simmering water. Simmer until melted and the mixture is very warm to the touch, about 3-4 minutes, stirring frequently. Remove from heat.

Stir in the sugar until combined. The mixture will appear grainy.

Add the eggs one at a time, stirring vigorously after each addition, then stir in the vanilla.

Add the flour, cocoa powder and salt and stir until incorporated, about 30 strokes. The batter will be glossy and pull away from the sides of the bowl.

Pour the batter into the prepared pan and spread evenly.

Place the peanut butter in a small bowl and heat for about 20 seconds in the microwave. Dollop on top of the brownie batter in straight lines, then use the tip of a butter knife to run down the center of the dollops from top to bottom, then left to right to create swirls.

Bake for 20-25 minutes until set, the top is shiny and a toothpick inserted into the center comes out with just moist crumbs. Cool completely on a wire rack, then remove from the pan and cut into squares. Enjoy!