Special Equipment: 4 (6 to 8-ounce) individual serving dishes
Melt the chocolate over a double boiler setup.
While the chocolate is melting, in a large mixing bowl, combine the egg yolks and sugar and beat, with a hand-held mixer, until very pale and thick and have doubled in size. Whisk in the coffee liqueur. When the chocolate has melted, whisk it into the egg yolk mixture.
In a medium bowl, whip the heavy cream to medium peaks. Using 1/3 of the whipped cream at a time, fold the cream into the chocolate mixture. Repeat this process until all the cream is incorporated. Transfer the mousse to individual serving dishes and chill for at least 1 hour.