2 cups bittersweet chocolate, finely chopped
2 tablespoons (1 ounce) unsalted butter, diced
2 tablespoons espresso or very strong coffee
1 cup cold heavy cream
3 large eggs, separated
1 tablespoon sugar
Whip the cream to soft peaks, then refrigerate to use later.Combine the chocolate, butter, and espresso in the top of a double boiler over hot, but not simmering, water, stirring constantly till its smooth.
Remove from the heat and let it cool until the chocolate is just slightly warm and all ingredients have melted to a liquid. If it is too cool, the mixture will seize when the other ingredients are added.
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 shot glasses. Refrigerate for at least 3 to 5 hours before serving. You can sprinkle grated chocolate ontop for serving.