Whip the cream(with an electric mixer, who do you think you are Julia Child?) to soft peaks, then refrigerate.
Combine the chocolate, butter, and espresso in the top of a double boiler over hot water, stirring frequently until smooth. Remove from the heat and let cool
Once the melted chocolate has cooled slightly, whip the egg whites in a medium bowl until they are foamy and beginning to hold a shape(this is where I messed up, really whip the whites, it takes awhile but gives the mousse its firmness). Sprinkle in the sugar and beat until soft peaks form.
Stir in the yolks to chocolate then add about one-third of the whipped cream. Fold in half the whites just until incorporated, then fold in the remaining whites, and finally the remaining whipped cream.
*Lots of mousse recipes recommend refrigerating it for 8 hours, but I think that if you can wait 8 hours clearly you don't like chocolate that much. Refrigerate it for awhile and then go ahead, its moudding, enjoy!