1) To make the syrup: Stir the syrup ingredients over medium heat until the sugar dissolves. Bring to a full rolling boil, remove from the heat, and set aside to cool.
2) To make the mousse: Whip the cream to soft peaks. Dissolve the mousse powder in the water, and immediately stir about 1 cup of the whipped cream into the mixture. Whisk the mousse mixture into the remaining whipped cream.
3) To assemble: Remove the top (browned) layer from the cake. Cut the cake in half horizontally. Place one layer into a springform pan that's been lined with a piece of parchment or waxed paper.
4) Sprinkle the cake with 3 tablespoons of syrup.
5) Spread 2 cups of filling onto the cake. Top with the second piece of cake.
6) Sprinkle with 2 tablespoons coffee syrup; let it sit for several minutes, then top with the remaining mousse. Refrigerate for several hours.
7) Run a thin spatula around the edge of the pan. Remove the springform. Move the cake to a serving platter.
8) Sprinkle with cocoa and/or chocolate shavings. Serve cold. Refrigerate any leftovers.
Yield: about 16 servings.