In the top of a double boiler set over simmering water combine the chocolate and the espresso. Heat the mixture, stirring, until just melted. Remove the pan from the heat and whisk in the butter, a little at a time, until smooth.
In a bowl with an electric mixer beat the yolks with 2/3 cup of the sugar until the mixture ribbons. Beat in the coffee-flavored liqueur. Set the bowl over a pan of gently simmering water and beat for 3-4 minutes, or until mixture is foamy and very hot to the touch. Beat over cold water for 3-4 minutes until mixture is cool and forms a ribbon. Mixture should have consistency of mayonnaise. Whisk yolk mixture into chocolate mixture.
In a bowl with an electric mixer beat the cream with the remaining sugar until it is firm but not dry. Stir one-fourth of the whipped cream into the chocolate mixture and gently but thoroughly fold in the remaining whipped cream.
Transfer the mousse to a serving bowl, serve immediately or chill it, covered, briefly. Serve with Creme Anglaise or whipped cream. Drink: Coffee
Adapted from Julia Child's recipe by David Rosengarten