PREPARATION:

1. Preheat the oven to 250°F/120°C. In a medium saucepan, combine the cream, milk, sugar and vanilla. Whisk to blend and bring to a boil over medium heat. Add the chopped chocolate and whisk until all the chocolate has melted.  Remove from the heat.
2. In a large bowl, lightly beat the egg yolks. Then, in a slow, steady stream, add the chocolate-cream mixture, whisking until smooth. Divide the mixture among 6 ramekins or small custard bowls (about 6 ounces each), and place them in a large high-sided baking dish or baking pan. 
3. Make a bain-marie, or water bath, by pouring cold water into the baking dish so that it comes halfway up the sides of the ramekins or custard bowls. Cover tightly with foil and bake on the center rack for 50 minutes to 1 hour, or until the custards jiggle slightly in the center when gently shaken (the custards will thicken as they chill).
4. Let cool to room temperature and then refrigerate until cold.  Serve cool.