Espresso Chocolate Mousse
12 ounces bittersweet chocolate, coarsely chopped
6 tbsp unsalted butter
2 tbsp pure vanilla extract
6 large eggs, separated
3/4 cup plus 2 tablespoons granulated sugar
1/4 cup plus 2 tablespoons espresso or strong brewed coffee
1/2 teaspoon cream of tartar
2-1/2 cups heavy cream
1-1/2 tbsp confectioners' sugar
In a medium saucepan, melt the chocolate with the butter over moderately low heat, stirring occasionally. Remove from the heat and stir in the vanilla.
In a medium, heatproof bowl, whisk the egg yolks with 1/4 cup plus 2 tablespoons of the granulated sugar and the coffee. Set the bowl over a saucepan of simmering water and whisk constantly until the yolks are frothy, about 6 minutes. Remove from the heat and fold in the melted chocolate.
In another medium bowl, whisk the egg whites with the cream of tartar and the remaining 1/2 cup of granulated sugar. Set the bowl over the saucepan of simmering water and whisk the whites constantly until an instant-read thermometer inserted in them registers 140°, about 5 minutes. Remove the bowl from the saucepan and continue beating the whites until stiff and glossy. Fold the beaten whites into the chocolate mixture.
In a clean bowl, beat 1 cup of the heavy cream until firm. Fold the whipped cream into the chocolate mixture until no streaks remain. Spoon the mousse into 8 glasses and chill for at least 2 hours.
Just before serving, beat the remaining 1-1/2 cups of heavy cream with the confectioners' sugar until firm. Dollop the cream over the mousse and serve.