Combine the chocolate, butter, and coffee in the top of a double boiler over hot, but not simmering, water, stirring frequently until smooth
Remove from the heat and let cool until the chocolate is just slightly warmer than body temperature
Meanwhile, whip the cream to soft peaks, then refrigerate
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. Sprinkle in the sugar and beat until soft peaks form
When the chocolate has reached the proper temperature, stir in the yolks. Gently stir in 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
Spoon or pipe the mousse into a serving bowl or individual dishes. Refrigerate for at least 8 hours