1. Allow the butter to soften at room temperature. 2. In a pan, slowly bring the cream to a near boil. 3. Cut the chocolate into small pieces and place them in the hot cream. 4. Stir the mixture well until the chocolate has disolved completely. 5. Allow this mixture to cool down. 6. Add the soft butter to the chocolate mixture and stir well. 7. Then add the egg yolks slowly, and again mix well. 8. In a deep bowl, beat the egg whites on high speed for about 10 mins until fully risen. 9. Once they are fluffy, add the sugar and beat again for a minute. 10. Remove a third of the egg white quantity and gently Fold it into the chocolate mixture with a spatula. 11. Once that is well incorporated, add another third and fold in the same way. 12. Place the remaining third in the mixture and do the same. 13. Spread the mixture evenly in a serving bowl, and place it in the freezer for 2 hours or until firm. Check periodically to make sure the mousse doesn't freeze.