Preheat your oven to 350°F/180°C. Grease or line an 8x8 inch (20x20 cm) baking pan with parchment paper, leaving an overhang for easy removal.
Into a heat-proof bowl add butter, dark chocolate, cocoa powder, and peanut butter. Place the bowl over a saucepan or bowl with 1-2 inches of hot water. Stir occasionally till melted completely. Remove from heat and set aside to cool.
In a mixing bowl, whisk together both the sugars and eggs until ribbon like texture is achieved with pale colour.
Add the melted butter to the sugar mixture. Add vanilla extract and stir with a spatula until smooth.
Sift in all-purpose flour into the wet ingredients. Add chopped peanuts and dark chocolate. Stir until just combined. Be careful not to overmix.
Pour the brownie batter into the prepared baking pan, spreading it out evenly.
Drop spoonfuls of melted peanut butter onto the brownie batter in the pan.
Use a knife or toothpick to gently swirl the peanut butter into the brownie batter to create a marbled effect. Add some chocolate chips over the top.
Bake in the preheated oven for 25-30 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs.
Allow the brownies to cool completely in the pan.
Once cooled, slice into squares and enjoy your delicious peanut butter brownies!