Pre-heat your oven to 370℉/180℃. Line an 8x8" square baking pan.
In a small bowl, break up the dark/semi-sweet chocolate and melt in the microwave in 20 second intervals, stirring in between to make sure it doesn't burn. Alternatively, set up a double boiler. Do this by placing a heatproof bowl over a small pot of simmering water. Add the chocolate to the heatproof bowl and stir until melted.
In a large mixing bowl, whisk together melted butter and both sugars. Add in the eggs and vanilla and mix to combine.
Now, add the melted chocolate slowly and combine once again.
Add the flour and cocoa powder, fold gently into the wet mixture with a rubber spatula or wooden spoon until just combined.
Pour half the brownie batter into the prepared pan. Spread evenly so the base of the pan is entirely covered. Top with a 4x4 arrangement of the Oreos.
Pour over the remaining brownie batter. Swirl in the peanut butter using a knife or small spoon (you may need to loosen up the peanut butter by warming in the microwave for 20-30 seconds to make it easier to swirl).
Bake 25 minutes. Remove from the oven and while the brownie is still warm, scatter over the roughly chopped peanut butter candy and press in just a little. Let cool completely before slicing and serving.