Pre-heat your oven to 180℃ Fan/370℉. Line an 8" Square Pan with greaseproof paper.
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. 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 mixing bowl, whisk together melted butter and both sugars. With an electric/stand mixer, go for a solid 3-5minutes so it is totally combined. Add in the eggs and vanilla and mix again.
Now, add the melted chocolate slowly and combine once again.
Add the flour and cocoa powder, fold gently into the wet mixture with a rubber spatula or wooden spoon until just combined. Pour into the lined tin.