In saucepan over medium heat place sugar, cornstarch, cocoa, and salt. Stir constantly until thick and bubbly.
Boil one minute and remove from heat.
Stir one half of the hot mixture into beaten egg yolks and then return all of the mixture to the pan.
Heat to a gentle boil. Cook and stir for one minute. Remove from heat and stir in butter and vanilla.
Pour into prebaked pie crust. Press plastic wrap onto top of filling.
Refrigerate 4-6 hours or until set. Store in refrigerator.