in Saucepan, combine sugar and salt. Chop up unsweetened chocolate and add with milk; gradually stir into dry mixture. Cook and stir over medium heat till mixture boils and thickens. Cook 2 minutes longer. Remove from heat. Stir small amounts hot mixture into yolks; return to hot mixture; cook 2 minutes, stirring constantly. Remove from heat.
Add butter and vanilla; cool to room temperature.(To prevent a crust from forming, put clear plastic wrap or waxed paper directly on top, touching surface of the hot pudding clear to sides of bowl.) Pour into baked pastry shell.
Meringue; Beat 3 eggs with 1/4 t cream of tartar and 1/2 t vanilla till soft peaks form. Gradually add 6 T sugar, beating till stiff peaks form and all sugar is dissolved. Spread atop pie, sealing to pastry. Bake in moderate oven 350 deg. about 12 to 15 minutes, or till meringue is golden.