Finely chop chocolate

Dice butter

Separate eggs

Whip the cream to soft peaks, then refrigerate

Combine the chocolate, butter, and espresso in the top of a double boiler over hot, but not simmering water, stirring frequently until smooth

Remove from heat and let cool until the chocolate mixture is just slightly warmer than body temperature.

Once the chocolate mixture has cooled, whip the egg whites in a medium bowl until they are foamy and beginning to hold a shape.

Sprinkle in the sugar and beat until soft peaks form.

When the chocolate mixture has reached the proper temperature, stir in the yolks.

Gently stir in about one-third of the whipped cream.

Fold in half of the egg whites just until incorporated, then fold in the remaining egg whites, and finally the remaining whipped cream.

Spoon or pipe the mousse into a serving bowl or individual dishes.

Refrigerate for at least 8 hours.