Preheat oven to 370℉/180℃. Line a 8″ square baking tin with greaseproof paper.
Melt the dark/semi-sweet 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. 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 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 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 the batter into the prepared pan.