In a small bowl, whisk together the melted butter and cocoa powder until smooth, set aside.
In a large bowl, whisk together the eggs and sugar until pale and thick. Stir in the vanilla and salt.
Stir the cocoa and butter mixture into the egg mixture. Fold in the flour, chocolate chunks and peanut butter chips. Spread evenly in the prepared pan.
Spoon the filling into a bag, cut a bottom corner off the bag and pipe wide lines of filling on top of the brownie batter. Use a butter knife to swirl the filling into the batter.
Place the pan in the oven and bake for 30 - 40 minutes or until just set. Do not over bake - these should have a soft fudgy texture reminiscent of the interior of a molten cake, but sturdy enough to cut. Remove from oven and cool completely.