1. Prepare crust. Combine cocoa, sugar, cornstarch and salt in medium saucepan. Gradually add milk to dry ingredients, stirring until smooth. Cook over medium heat, stirring constantly, until mixture comes to a boil; boil l minute.
2. Remove from heat; stir in butter and vanilla. Pour into prepared crust. Carefully press plastic wrap directly onto pie filling. Cool; refrigerate 6 to 8 hours. Top with whipped cream. Cover; refrigerate leftover pie. 6 to 8 servings.