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, cocoa powder, chopped milk chocolate until just combined. Pour into the lined tin.
In a small bowl combine the topping ingredients (make sure the butter is fully melted). Pour this over the brownie batter.
Bake 25 minutes - then remove from the oven, top with foil to prevent the topping from baking, then bake a further 10-15 minutes - until this topping is only barely wobbling.
Let cool before slicing and serving - some of the topping may crumble a little! Optional: melt 25g milk chocolate and drizzle over for decoration.