Position rack in center of oven and preheat to 300F. Whisk egg yolks and 1/4 cup sugar in medium bowl. Bring cream and remaining 1/4 cup sugar to simmer in heavy medium saucepan. Reduce heat to low. Gradually add chopped chocolate to cream mixture and whisk until smooth. Gradually whisk hot chocolate mixture into yolk mixture. Mix in vanilla.
Ladle custard into four 10-oz. custard cups (or creme brulee cups). Place cups in large baking pan. Add enough hot water to pan to come halfway up sides of cups. Bake until custards are set in center, about 1 hour. Remove custards from water and cool. Cover and refrigerate overnight.
Preheat broiler. Sprinkle 1/2 tablespoon sugar over each custard. Broil until sugar caramelizes, watching carefully, about 2 minutes. Serve hot, or refrigerate up to 1 hour and serve cold.