Bring a pan with water almost to a boil.
Hang a bowl in the pan, while not touching the hot water.
Break the chocolate into pieces and melt the chocolate in the warm bowl (this method is called au Bain-Marie).
When the chocolate has melted, remove the bowl from the water bath and put it at room temperature (about 5 minutes).
Optional: stir the liquor into the chocolate with a spatula after 5 minutes.
Put the cream, mascarpone, and sugar into a bowl and whisk until you get soft peaks.
Spoon about ⅓ of the melted chocolate into the bowl the cream mixture and fold it gently with a spatula.
Fold the rest of the chocolate into the cream.
Spoon the chocolate mousse in the glasses and put the mousse in the refrigerator.
Put it in the refrigerator for at least 2 hours (stiffen) before serving.
