Pre-heat your oven to 370℉/180℃. Line an 8x8" 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 until fully combined (3-5 minutes). Add in the eggs and vanilla and whisk again.
Now, add the melted chocolate slowly and combine.
Add the flour, salt and cocoa powder. Fold gently with a rubber spatula or wooden spoon until just combined. Pour half the batter into the lined pan.