Break the chocolate into pieces and place in a large bowl. Add in the coffee and butter and melt everything gently over a pot of boiling water.
When everything is melted, remove the bowl from the heat and stir in the rum. Beat in the eggs one by one, while the mixture is still warm. Then whisk the egg whites until they are thick and firm. Take one large spoonful to fold into the chocolate first and then fold the rest in gently. Pour the mixture into 6 ramekins, cover and chill in the fridge.
Mascerate some mandarin segments in the orange liqueur and serve with the chocolate mousse with some whipped cream.