Break the dark chocolate into small chunks and tip into a bowl with the coffee, rum. Bring 250ml double cream to the boil, pour over the chocolate mixture and stir until the chocolate has melted and the mixture is smooth. Divide the mix between 6 coffee cups and leave overnight in the fridge.
To serve, whip the remaining double cream with the caster sugar until just set. Spoon the cream onto the chocolate pots and serve dusted with cocoa powder so each cup looks like a cappuccino.