Pre-heat your oven to 200c/180c Fan. 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, go for a solid 3-5minutes so it is totally combined. Add in the eggs, one at a time and mixing in between.
Now, add the melted chocolate slowly and combine once again.
Add the flour, cocoa powder and a pinch of salt - 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. Use the spoon (or clean, wet hands) to press down just a little into an even layer.