Preheat the oven to 350°F (175°C) and line a 9×13-inch baking pan with parchment paper.
In a large bowl, whisk together the melted butter, granulated sugar, and brown sugar.
Add the eggs and vanilla extract, whisking until smooth.
Mix in the cocoa powder.
Fold in the flour, baking powder, kosher salt, and chocolate chips until just combined.
Spread the brownie batter evenly into the prepared pan.
In a small bowl, stir together the peanut butter, powdered sugar, and melted butter until smooth.
Drop spoonfuls of the peanut butter mixture over the brownie batter.
Use a butter knife or skewer to gently swirl the peanut butter into the batter without overmixing.
Bake for 30 to 35 minutes, or until a toothpick inserted near the center comes out with a few moist crumbs.
Cool completely before slicing into squares.
