For the chocolate mousse, whip the mascarpone in a bowl and place in the freezer for about 15 minutes. Melt the cooking chocolate over a water bath. Whip the cream until very stiff.
Stir the cooled chocolate by the spoonful into the very cold, not yet frozen, mascarpone until smooth, add the 2 tablespoons of rum. Fold in the stiffly whipped cream with a whisk and pour the mixture into glasses.
Chill the chocolate mousse for 1 hour.