Pre-heat your oven to 180℃ Fan/370℉. Line an 8x8" square baking pan.
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. Add in the eggs, one at a time and mixing in between. Add the melted chocolate slowly and combine once again.
Add the flour, salt and cocoa powder, fold gently into the wet mixture with a rubber spatula or wooden spoon until just combined. Pour into the lined pan.
Bake 25-28 minutes, until just a little wobbly in the middle. Let cool.