1. Process chocolate until finely grated. Heat cream to just below boiling point, gradually add cream to chocolate while motor is operating; add rum, process until smooth. Transfer chocolate mixture to large bowl.

2. Beat egg whites is small bowl with electric mixer until soft peaks form, gradually add sugar a tablespoon at a time, beating well after each addition.

3. Fold egg white mixture through chocolate mixture in 2 lots.

4. Whip extra cream until soft peaks form, fold cream through chocolate mixture. Pour mousse into serving glasses. Refrigerate mousse several hours or overnight.