(Adapted from Bouchon. Read more in the post, Cookies and Chocolate Mousse.)
Makes 8 four-ounce servings.
4 1/2 ounces bittersweet chocolate (between 60% to 65%), 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
Combine the chocolate, butter, and espresso 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. To test, dab some chocolate on your bottom lip. It should feel warm. If it is too cool, the mixture will seize when the other ingredients are added.
Meanwhile, whip the cream to soft peaks, then refrigerate. In a medium bowl, whip the egg whites 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. (The mousse can be refrigerated for up to a day.)
If you do not have a true double boiler, place a metal bowl over a pot of simmering water, but be careful to not allow the bottom of the bowl to touch the water. Also be careful when handling the metal bowl as it will heat up some.
I used Callebaut chocolate (60.3%) that I found in bulk at Whole Foods. Whichever dark/bittersweet chocolate you use, do not go above 65% or the mousse will taste too bitter.