Gently heat bittersweet chocolate (chopped or discs/pieces) over a double boiler, stirring constantly until melted. Chocolate will be melted at 120 degrees. Reserve. In a non-reactive bowl, whip cream to soft peaks and reserve.
In a heavy-bottomed sauce pot over medium heat, combine sugar and water and heat to a soft-crack stage (282 degrees). In a stainless steel or copper bowl, beat yolks until thickened and then slowly pour sugar-water into yolks.
Gently fold whipped cream into yolk mixture. Once the whipped cream is fully incorporated, gently fold in the tempered chocolate, reserving 3 ounces for the drizzle. Pour mousse into individual bowls or one large, decorative bowl. Drizzle reserved chocolate over the top and refrigerate for a minimum of one hour. Mousse may be kept refrigerated for up to 48 hours.