Preheat your oven to 325°F. Lightly coat an 8×8-inch baking pan with nonstick cooking spray. Line the bottom and sides with parchment paper, leaving an overhang on opposite sides for easy removal later.

In a small saucepan over medium heat, combine the unsalted butter and granulated sugar. Turn off the heat, then add the chocolate chips and stir until melted and smooth. Add to a large bowl and set aside to cool slightly.

Add the eggs and vanilla to the cooled chocolate mixture. Using a whisk (paid link), beat vigorously for 2 minutes until fully incorporated and glossy.

Using a spatula (paid link) or wooden spoon, add the unsweetened cocoa powder to the bowl and fold it in until fully incorporated. Gently fold in the all-purpose flour, sea salt, and baking soda just until no dry flour streaks remain. Do not overmix.

Pour the brownie batter into the prepared baking pan and smooth the top evenly with your spatula (paid link).

Warm the creamy peanut butter in the microwave for 10 to 15 seconds until slightly fluid. Drop generous spoonfuls of peanut butter across the batter. Use a butter knife or offset spatula (paid link) to gently drag and swirl the peanut butter through the top layer of batter.

Bake for 35 to 40 minutes, or until a toothpick inserted into the center comes out with a few moist, fudgy crumbs (not wet batter).

Remove from the oven and let the brownies cool completely in the pan on a wire rack for at least 30 minutes. Lift out using the parchment overhang and slice into clean squares.