Pre-heat your oven to 180℃ Fan/370℉. Line a 12 hole muffin tin with muffin cases.
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.
Melt the butter with the instant coffee so that it can dissolve. Do this either in the microwave or in a small saucepan.
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. Scoop into the muffin tin holes, they don't rise much at all so use all the batter!
Bake 20-25 minutes, until no longer wobbly. While still warm, create the indents by using a teaspoon measure, egg cup or end of a wooden spoon. Press into the middle, leaving tall sides. Let cool.