Pre-heat your oven to 370℉/180℃. Line 12 muffin holes with cupcake/muffin liners.
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, combine for 3-5 minutes. Add in the eggs, one at a time, mixing in between.
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.
Scoop a generous tablespoon of batter into each muffin cup.