Melt the coarsely chopped couverture in a hot water bath, set aside.
Beat the egg and yolk in a metal bowl in a hot water bath until frothy. Stir in the couverture and brandy or rum.
Let cool down lukewarm. In the meantime, whip the cream until it is half stiff and fold in. Pour the mousse into a bowl and cover and chill for 2 hours before serving.