Preheat oven to 350°F (175°C) and line an 8x8 inch baking pan with parchment paper.
In a large mixing bowl, combine 1/2 cup melted butter, granulated sugar, and brown sugar until well incorporated.
Whisk in the eggs and vanilla extract until the mixture is smooth and slightly glossy.
Gently fold in the flour, cocoa powder, and salt until just combined; do not overmix.
Stir in the chocolate chips and spread the batter evenly into the prepared baking pan.
In a separate small bowl, whisk together the peanut butter, powdered sugar, and 2 tablespoons melted butter until smooth.
Dollop the peanut butter mixture over the brownie batter and use a butter knife to create a marbled swirl pattern.
Bake for 30 to 35 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs attached.
Remove from oven and allow the brownies to cool completely in the pan before slicing into 16 squares.