Whisk egg yolks. Bring cream and sugar to a simmer in a saucepan. Reduce heat to low. Gradually add chocolate and whisk until smooth. Gradually whisk hot chocolate mixture into egg yolks. Mix in vanilla. Ladle into 6 custard cups. Place cups in a pan and surround cups with hot water. Bake for one hour. Cool. Preheat broiler. Sprinkle brown sugar on tops of cups and broil 2 minutes.