How to Make It
In the top of a double boiler, combine the chocolate, butter and espresso and cook over simmering water over moderately low heat, stirring, until the chocolate is melted. Remove the top of the double boiler and let cool to 75° on an instant-read thermometer. Beat in the egg yolks until incorporated.
In a large bowl, beat the egg whites at medium-high speed until very soft peaks form. Gradually add the sugar and beat until the whites are slightly firm and glossy.
In another bowl, beat the cream until firm. Gently fold half of the whipped cream into the chocolate mixture, then fold in half of the beaten whites until no streaks remain. Repeat with the remaining whites and whipped cream.
Spoon the chocolate mousse into glasses or bowls and refrigerate until chilled, at least 3 hours. Garnish with chocolate shavings and serve chilled.