Crust: Combine all the ingredients in a bowl and toss until thoroughly combined. Press the mixture into the bottom of a 10-inch spring form pan and refrigerate.
Dense Mousse: Melt the semisweet chocolate and the butter together in the top of a double boiler over medium-low heat. Let cool slightly, then whisk in the egg yolks. Whisk in the malt powder. Keep warm. In the bowl of a mixer, whip the egg whites until they form soft, droopy peaks. Still whipping, add the sugar in a thin stream and whip until the mixture is stiff and glossy. Fold into the chocolate mixture. Pour into the pan and smooth the top with a rubber spatula. Refrigerate.
Light Mousse: Melt the chocolates together in the top half of a double boiler over medium-low heat. Let cool slightly. Meanwhile, in a mixer, whip the cream until it forms soft peaks and chill. In a mixer, whip the egg whites until they form soft, droopy peaks. Still whipping, add the sugar in a thin stream and whip until the mixture is thick and glossy. A third at a time, fold the egg whites into the chocolate mixture. (You may need to whisk, not fold, the first third if the mixture is very thick).
Fold the whipped cream into the mousse. Pour over the dense mousse and smooth the top with a rubber spatula. Chill until set, at least 3 hours or up to 1 day. To turn the pie out of the pan, warm the sides with a torch and remove the walls. Slice with a knife dipped in hot water and serve with warm chocolate sauce, if desired.
Combine chocolate, place over double boiler, stir until melted. Whisk in syrup and water, incorporating well. Serve warm.
Recipe courtesy Gale Gand, American Brasserie by Gale Gand, Rick Tramonto, Julia Moskin: Clarkson N. Potter Publishers, 1997