Note: These recipes have been adapted from "Desserts by Pierre Herme," by Dorie Greenspan. Copyright 1998 by SOCREPA and Dorie Greenspan. Reprinted by permission of SOCREPA. Published by Little, Brown.
To make the coffee syrup: Spoon off 2 tablespoons of the simple syrup, cover, and refrigerate to use later for the cocoa-dusted almonds. Keep the remainder of the syrup at the ready. Line a strainer with a double thickness of dampened cheesecloth, and place it over a small bowl; set aside.
In a medium saucepan, bring 2 cups of water to a boil. Add the ground coffee, stir, and immediately pour the mixture through the lined strainer. You should have about 3/4 cup of very dark coffee. Stir in the instant coffee and the simple syrup. Reserve. (The coffee syrup can be made up to a week ahead and kept covered in the refrigerator.)
To make the mousse: Beat the cream until it holds medium-firm peaks, then cover and chill. In the bowl of an electric mixer fitted with the whisk attachment, beat the eggs and egg yolks at the lowest speed for a few seconds, just to break up the eggs; set aside while you prepare the chocolate and sugar syrups.
Melt the chocolate in a microwave oven or in a metal bowl set over a pan of simmering water. Remove the chocolate from the heat, and, if necessary, pour it into a bowl large enough to hold all the ingredients for the mousse. Cool the chocolate until an instant-read thermometer registers 114 degrees.
While the chocolate is melting and cooling, place the sugar and water in a small, heavy-bottomed saucepan. Bring the mixture to a boil, stirring occasionally and washing down the sides of the pan with a pastry brush dipped in cold water. Boil the syrup over high heat without stirring until an instant-read thermometer registers 257 degrees, about 8 to 10 minutes. Immediately remove it from the heat.
With the mixer on the lowest speed, beat the eggs for a few seconds, then very slowly add the hot sugar syrup in a thin, steady stream. To avoid splatters, try to pour the syrup down the side of the bowl, not into the spinning whisk. (Inevitably, some will splatter, but don't attempt to scrape the hardened syrup into the eggs -- you'll get lumps.) Increase mixer speed to high, and beat eggs for about 5 minutes, or until they are pale and have more than doubled their original volume. If the mixture is still warm, reduce the speed to medium and continue to beat until eggs are at room temperature.
Using a large rubber spatula, fold about 1/4 of the whipped cream into the chocolate. Fold in the rest of the cream, and then, very delicately, fold in whipped egg mixture.
To make the ganache: Place the chocolate and cocoa in a medium bowl. Bring the cream to a boil, and remove it from the heat. Pour the cream into the chocolate in three additions, using a rubber spatula to stir the mixture in concentric circles, starting each time with a small circle in the center of the bowl and working your way out into larger circles. You'll have a smooth, glossy ganache. Allow the ganache to rest uncovered and undisturbed (don't stir it) at room temperature until it sets, 40 to 60 minutes, depending on the room's temperature. When the ganache is properly set, it will hold a ribbon for a second or two when stirred. It is ready to be used now or covered and refrigerated until needed. If the ganache is chilled, it must be brought back to its proper consistency by leaving it at room temperature until it's spreadable (the best method), or by heating it over hot water or in a microwave oven at low power. Do not beat it or otherwise overwork it, or it will lose its lovely -- and characteristic -- sheen.
While the ganache is setting up, transfer the cake from the refrigerator to the counter. (Working on a cold rather than a solidly frozen cake will facilitate applying the ganache.)
To make the cocoa-dusted almonds: Center a rack in the oven and heat the oven to 325 degrees. Sprinkle the almonds with the cocoa, then toss with the simple syrup to coat, and spread them out on a parchment-lined baking sheet. Toast the almonds, stirring every 3 to 4 minutes, until they are deeply and evenly browned, about 10 to 12 minutes. Transfer the sheet to a rack, and cool the almonds to room temperature. (The almonds can be used immediately or packed into an airtight container and stored at room temperature for about 4 days.)
To assemble the cake: Trim the top of the genoise so it is nice and smooth. Cut two layers from the cake, each between 1/4- and 1/2-inch thick (these layers need to be extremely thin), and set them aside. The remainder of the genoise can be frozen for later use.
Center an 8 3/4-inch dessert ring on a cardboard cake round. Place one layer of genoise, cut-side up, in the ring. Brush enough coffee syrup over the cake to moisten it thoroughly. Using an offset spatula or a flexible rubber spatula, spread 2 to 2 1/2 cups of mousse over the cake and smooth the top. Position a second layer of cake on the mousse, pressing down gently and jiggling the cake to settle it in place. Brush on some coffee syrup, and spread on another 2 to 2 1/2 cups of mousse; smooth the top so it is level with the edge of the dessert ring. (You may have some syrup left over, and you will have extra mousse. The syrup can be refrigerated for another use, and the mousse can be refrigerated for two days or frozen for a month.) To set the cake, transfer to the freezer and freeze for 2 hours. (The cake can be made to this point and frozen for up to a month.) If you're not going to freeze the cake for long-term storage, it should be transferred to the refrigerator to defrost after 2 hours.
Remove the dessert ring. Using a long metal offset spatula, spread ganache over the top and sides of the cake. If you want to repeat with another one or two layers of ganache, go ahead -- you've got plenty of ganache to play with. If the ganache is firm enough to hold a design, you can decorate the top now. If the ganache is still very soft, return the cake to the refrigerator for a few minutes to set it enough to hold a line drawn across its surface with a knife. (Keep the leftover ganache at hand so you can cover up any design attempts that don't pass muster.)
Decorate the top of the cake using the blade of a long, serrated bread knife. Hold the handle of the knife with one hand and the tip with the other. Starting at one edge of the cake and holding the knife almost perpendicular to the cake, gently slide the knife from one side of the cake to the other. Without losing your place at the edge of the cake, shift the blade about 1/16 inch and slide it 1/16 inch back to the opposite edge of the cake. You will have created the first V in a herringbone pattern. Continue until you have decorated the entire top of the cake. If the knife blade becomes clogged with ganache, clean the blade before continuing the pattern. Return the cake to the refrigerator for a few minutes to set the design. If you prefer, the cake can be finished with a flat-top -- that is, a smooth coating of ganache -- or you can decorate the top with swirls, ridges, or any other pattern that pleases you.
The last step is to press the toasted almonds against the sides of the cake. (If the ganache has set so it's very firm and the almonds won't stick to the sides, just warm the sides ever so slightly with a hair dryer -- lightly melting the ganache will help the almonds to adhere.) The cake can be served now or chilled until serving time. If the cake is very cold and firm, let it rest at room temperature for 15 to 30 minutes before serving.