Preheat the oven to 180°C. In a bain-marie, melt the dark chocolate, 150g of milk chocolate, 75g of peanut butter, and butter together. Once melted, let the mixture cool slightly.
In a separate bowl, beat the eggs with the caster sugar until the mixture lightens slightly in color, being careful not to overmix.
Fold the melted chocolate mixture into the beaten eggs until well combined.
Add the flour to the mixture and stir until smooth. Then, fold in the remaining chocolate chips.
Take about 200g of the batter and mix in the remaining 150g of peanut butter until smooth.
Pour the remaining batter into a baking tray lined with parchment paper. Swirl the peanut butter mixture through the batter by dragging a knife or fork through it to create a marbled effect.
Bake the brownie in the preheated oven for 20 minutes. It should still be slightly wet in the center. After cooling, refrigerate the brownie overnight.
The next day, cut the brownie into squares and enjoy.