mix sugar, cornstarch, salt, and chocolate in saucepan

gradually stir in milk

cook over medium heat, stirring constantly until mixture thickens and boils

boil for one minute

gradually stir in half of hot mixure into slightly beaten egg yolks

blend egg yolk mixture back into saucepan

boil one more minute stirring constantly

remove from heat

blend in butter and vanilla

pour immediately into baked pie shell

finish with meringue and let cool