Pre-heat your oven to 370℉/180℃. Line an 8" Square Pan with greaseproof paper.
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. With an electric/stand mixer, go for a solid 3-5minutes so it is totally combined. Add in the eggs, one at a time and mixing in between.
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. Fold in the peanuts. Pour into the lined tin.
Place the peanut butter in a small bowl in the microwave for 20 seconds to loosen it. Use a spoon to drizzle this over the brownie batter. Use a spoon to add dollops of marshmallow fluff over the top.
Bake 30 minutes until the top is shiny and it's still just a little wobbly in the middle. Let cool before slicing and serving.
