Meanwhile, in 2-quart saucepan, mix granulated sugar and cornstarch. Gradually stir in milk; cook over medium heat until mixture boils, stirring constantly. Reserve 1 tablespoon chocolate chips for topping; coarsely chop. Add remaining chocolate chips and vanilla, stirring until melted and smooth. Pour into medium bowl; cover surface with plastic wrap. Refrigerate about 30 minutes to chill.