Place chocolate and 1 cup of the cream in a saucepan. Stir over gentle heat or microwave at 50% power for 2 minutes until chocolate is melted. Cool.
Whisk egg whites in a clean bowl with sugar until stiff. In another bowl whisk remaining cream with vanilla until it forms firm peaks. Fold a third of the egg whites into the cooled chocolate to lighten the mixture. Fold in remaining egg whites then the cream.
Chill until quite firm, about 4 hours or up to 24 hours, before serving. Scoop mousse into cups to serve. Prepared mousse can be covered and chilled for up to 48 hours.
