Preheat your oven to 350°F (175°C) and line an 8x8 inch baking pan with parchment paper.
In a large bowl, whisk together the melted butter and granulated sugar until combined.
Add the eggs one at a time, beating well after each addition.
Stir in the cocoa powder, flour, and salt until just combined.
In a separate bowl, mix the peanut butter and powdered sugar until smooth and creamy.
Pour half of the brownie batter into the prepared pan and spread evenly.
Drop spoonfuls of the peanut butter mixture onto the brownie layer and smooth out the layer.
Pour the remaining brownie batter over the peanut butter layer and spread evenly.
Bake in the preheated oven for 25-30 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs.
Let the brownies cool completely in the pan on a wire rack before cutting and serving.