In saucepan, combine sugar, cornstarch, and salt; gradually stir in milk and chocolate.
Cook and stir over medium heat ‘til bubbly. Cook and stir 2 minutes, stirring constantly.
Remove from heat.
Stir small amount hot mixture into yolks; immediately return to hot mixture; cook 2 minutes, stirring constantly. Remove from heat. Add butter and vanilla. Pour into cooled baked pastry shell. Spread meringue atop pie and bake at 350 degrees for 12 to 15 minutes. Cool.
Beat egg whites with vanilla and cream of tartar ‘til soft peaks form. Gradually add sugar, beating ‘til stiff and glossy peaks form and all sugar is dissolved. Spread meringue over hot filling, sealing to edge of pastry. Bake as above.
NOTES: I use the Pillsbury ready-made pie crusts. I melt the chocolate squares in the microwave in a little Pyrex dish. I heat the milk up a little, too, in the microwave and then it doesn’t take quite as long on the stovetop.