Preheat the oven to 200C/180C Fan/Gas 6. Line an 8 inch square tin. Melt the dark chocolate in the microwave in 20 second intervals. Set aside.
In a large bowl/the bowl of a stand mixer, combine the melted butter and both sugars. Add the eggs and mix again. Slowly incorporate the melted chocolate until everything is fully combined.
Fold in the flour and cocoa powder until just combined. Pour the batter into the tin and bake for 30 minutes. Let cool about 20 minutes before making the topping.
Make the filling: combine the butter, peanut butter and icing sugar until fluffy (use an electric whisk/stick blender). Spread over the brownie.
Melt the dark chocolate for the topping in the microwave (checking at 20 second intervals). Once melted, pour over the brownies. Set in the fridge for at least an hour.
Slice with a hot knife for perfect lines!
