1. Place an oven rack on the middle shelf of the oven and the second rack on the top shelf. Preheat the oven to 200 degrees F. Line 2 baking sheets with parchment paper. On the first baking sheet, trace two 8-inch circles on the parchment paper, using the bottom or an 8-inch cake pan as a template. Trace a third circle on the second baking sheet. Turn the parchment paper over on the baking sheets so that you can see the outline but the meringue won't come in contact with the markings when you pipe it onto the parchment.
2. To make the meringue: In a large bowl, beat the egg whites and cream of tartar with an electric mixer at low speed until foamy. Increase the mixer speed to medium high and continue beating until soft peaks form. Continue beating, adding the superfine sugar 1 tablespoon at a time, until stiff, glossy peaks form. Beat in the vanilla.
3. Sift the confectioners' sugar, cocoa powder, and salt together. Using a rubber spatula, carefully fold the cocoa mixture into the meringue just until no streaks of cocoa remain. Do not fold the cocoa mixture in any more than is necessary, as overmixing will deflate the meringue. You want the mixture to hold stiff peaks, which ensures that the meringue disks will hold their shape.
4. Fit a 16- or 18-inch piping bag with a 3/8-inch (No. 5) plain tip. Fill the bag with the meringue. Starting from the center of each circle, squeeze out the meringue in a spiral. Pipe the remaining meringue into long strips on the baking sheet with 1 meringue circle.
5. Bake the meringues for 2 hours, or until crisp. If the oven seems too hot, or the meringues are beginning to color, reduce the oven temperature to 175 degrees F and prop the oven door open slightly with the handle of a wooden spoon. Turn the oven off and let the meringues cool in the oven for 1 hour. Cut the meringue strips into 1/4- to 1/2-inch pieces.
6. To make the chocolate mousse: Place the chopped chocolate in a medium bowl.
7. In a small saucepan, combine 3/4 cup of the cream with the coffee mixture. Heat over medium heat until bubbles start to form around the edges of the pan and the cream is just about to boil. Immediately pour the hot cream over the chocolate and let sit for about 30 seconds to soften the chocolate. Stir until smooth.
8. Let the chocolate mixture cool until it is at room temperature but still malleable. In a large bowl, beat the remaining 1 1/4 cups cream until soft peaks form. Using a rubber spatula, fold the whipped cream into the chocolate mixture until thoroughly blended.
9. To assemble: Place 1 meringue circle on a serving plate. With a small palette knife or offset spatula, spread one third of the mousse over the surface of the meringue. Cover with a second circle of meringue and spread with another one third of the mousse. Place the final meringue disk on top of the cake. Frost the top and the sides of the cake with the remaining mousse. Press the broken meringue sticks into the top and sides of the cake. Dust the entire cake with confectioners' sugar and cocoa powder. Refrigerate for at least 1 hour or up to 1 day before serving. The meringue will soften slightly the longer it sits in the refrigerator, but remains just as delicious.
Courtesy of Simple French Desserts by Jill O'Connor (Chronicle Books, 2000)