Combine cocoa, sugar, cornstarch and salt in a medium saucepan. Blend in milk until smooth.
Cook and stir over medium heat until mixture comes to a boil.
Boil and stir for 3 minutes. Remove from heat. Blend in butter and vanilla. Pour into pie shell. Carefully press plastic wrap directly onto pie filling. Chill 3 to 4 hours. Serve with sweetened whipped cream or topping and chocolate curls, if desired.
Note: Cook time is cooling time. You can also use a cooled crumb crust for shell.