In another medium bowl, beat egg yolks with fork; set aside. In 3-quart heavy saucepan, mix 2/3 cup sugar and the cornstarch. Gradually stir in milk. Heat to simmering over medium-high heat. Reduce heat to medium. Slowly stir half of hot milk mixture into egg yolks; return mixture to pan. Cook over medium heat about 5 minutes, stirring constantly, until mixture thickens and boils. Remove from heat. Immediately stir in chocolate and vanilla until chocolate is melted and mixture is smooth. Spoon filling into crust. Press plastic wrap on filling. Refrigerate at least 8 hours or until set.