Preheat your oven to 160℃ (140℃ for fan ovens) and line an 8x8-inch tin with baking paper.
Melt the dark chocolate and the butter together over low heat on the stove then set aside to cool for around 10 minutes.
Place the sugar and eggs into a large bowl. Use an electric whisk to whisk them together for a few minutes until it increases in volume and looks pale and silky.
Fold the cooled melted chocolate mixture into the bowl. Then stir in the flour and cocoa powder.
Gently mix the mini peanut butter cups and chocolate chips in then transfer the batter to your lined tin.
Drizzle the peanut butter on top of the batter and use a toothpick to swirl it around in a pattern. Top with extra peanut butter cup pieces if you like.
Bake for around 35 minutes until the brownies have risen and are pulling away from the tin slightly. There should be a very slight wobble in the middle.
Leave the brownies in the tin until cool then transfer them to the fridge overnight or for at least 6 hours to set. This will make them ultra fudgy and chewy.
Once set, remove them from the tin, cut into squares and enjoy!