Combine sugar, cocoa, cornstarch, egg yolks, and milk in medium size heavy saucepan. Whisk until all the ingredients are well blended. Cook over medium heat, STIRRING CONTINUOUSLY until mixture thickens. Remove from heat; add butter and vanilla. Pour mixture into a bowl. Press a piece of plastic wrap inside the bowl on top of the filling. Refrigerate until filling is cold.
Cream Cheese Layer: Combine softened cream cheese with powdered sugar. Beat until mixture is smooth. Add Cool Whip and vanilla. Beat until mixture is blended. Spread the mixture on the bottom of the cooled crust.
Spread the chocolate filling over the cream cheese layer. Refrigerate. Top with Cool Whip or sweetened whipped cream before serving.
Note: If you don't have time to prepare a pie crust, substitute a Keebler Ready Crust Shortbread or Graham Cracker Crust.