Preheat oven to 200C/180C Fan/Gas 6. Line a 8″ square baking tin with greaseproof paper.
Melt the dark chocolate by breaking up into small pieces in a bowl and heating for 30 second intervals in a microwave, stirring in between. Set aside to cool whilst making the brownies.
In a large bowl or the bowl of a stand mixer combine the melted butter, and both the brown and granulated sugars on a medium speed until totally combined.
Add the eggs and vanilla extract and mix again. Then check that the chocolate has cooled a little before adding in and combining. Add in the flour and cocoa powder and fold together. Pour into the tin.
Make the topping by combining all the ingredients with a whisk - ensure it is smooth! Pour over the top of the brownie batter and bake for about 40-45 minutes - until it is cracking and no longer wobbly (a small wobble in the middle is fine).