Chill a decorative glass bowl or 6 stemmed glasses.
Rinse and pick over cranberries and place in a large saucepan. Add sugar and cook over medium heat about 5 minutes, until cranberries have burst and the mixture has a thick jam-like consistency. Cool.
Stir in zest, juice and liqueur and rub through a strainer to remove skins.
Whip cream and fold 3/4 of it into cranberry puree. Place mousse in bowl or glasses. Decorate with reserved whipped cream just before serving.