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/the bowl of a stand mixer, combine the melted butter and both sugars. Add the egg and mix again. Slowly incorporate the melted chocolate until everything is fully combined.
Fold in the flour and cocoa powder. Pour this into the lined pan.