Mis cocoa, 3/4 cup sugar,cornstarch, and salt in a sauce pan. Gradually stir in milk mix until smooth. Mix until it thickens. Boil 1 minute. Stir half chocolate mix in to beaten egg yolks. Blend into hot mixture in pan. Boil 1 minute then remove from heat. Then, blend butter and vanilla. Pour in to baked pie shell.
Beat egg whites with cream of tartar until foamy. Add 4 tablespoons of sugar gradually. Beat until stiff peaks hold. Flavor with 1/2 teaspoon vanilla, Pile onto Chocolate mixture.
Bake until foam tips are golden brown. 8 to 10 minutes.