1. Heat milk and 1/3 cup of the sugar in medium saucepan until bubbles appear at edge.
2. Meanwhile, whisk remaining 1/3 cup sugar, cornstarch, yolks and vanilla in a bowl. Gradually whisk hot milk into yolk mixture. Return yolk mixture to saucepan. Cook over medium heat, stirring constantly, until mixture thickens and comes to a boil; boil 1 minute.
3. Remove from heat; immediately sprinkle chocolate over top and let stand 2 minutes. Whisk in chocolate and butter until smooth. Pour into four dessert bowls. Cover tops directly with plastic wrap. Refrigerate until cold, 3 hours. Makes 4 servings.