In medium saucepan, stir together cocoa, sugar, cornstarch and salt. Gradually add milk to dry ingredients, stirring constantly, until smooth. Cook over medium heat, stirring constantly, until mixture boils; boil for one minute. Remove from heat; stir in butter and vanilla. Pour into shell. Carefully press plastic wrap directly onto pie filling. Cool; refrigerate for 6 to 8 hours. Top with whipped cream.