Set a rack at the middle level of the oven and preheat to 300 degrees F.
Bring cream and 1/2 cup sugar to a boil in a nonreactive pan. Remove from heat, add chocolate and allow to stand 3 minutes to melt. Whisk until smooth.
Whisk egg yolks into a bowl, then whisk in chocolate mixture. Strain inot a 2-quart gratin dish that has been placed inside a baking dish or roasting pan.
Place pans on oven rack. Add warm water to the bottom pan to about half of the depth of the gratin dish. Bake about 1 hour, or until set. To test for doneness, insert a thin knife or wooden pick in the center of the custard; it should emerge clean. Remove baking dish from roasing pan (leave the pan of water in the oven to cool) and chill custard.
To caramelize the top, blot any moisture from the top of the chilled custard. Sprinkle evenly with 1/2 cup sugar. place under preheated broiler, or use blow torch to caramelize. Refrigerate until servign within several hours of caramelizing.