Preheat the oven to 200C/180C Fan/Gas 6. Line an 8" square baking tin.
In a small bowl, break up the dark chocolate and melt in the microwave in 20 second intervals, stirring in between to make sure it doesn’t burn.
In a large mixing bowl, whisk together butter and both sugars for 3-5 minutes. Add in the eggs, one at a time and mixing in between. Add the vanilla and mix again.
Now, add the chocolate slowly (check it first, to make sure it isn’t so hot that it will cook the eggs) and combine once again.
Gently fold in the flour, cocoa powder, milk chocolate and peanuts. Pour the batter into the prepared tin.
Put the peanut butter into a small microwaveable ramekin/bowl and place in the microwave for about 30 seconds (until runny). Drizzle the peanut butter over the batter and use a knife to swirl in.
Bake for 25-30 minutes. Let cool on a wire rack before slicing and serving.
