Grandma's Chocolate Meringue Pie

In a saucepan, mix sugar, cocoa, cornstarch and salt; gradually add milk. Cook and stir over medium-high heat until thick and bubbly. Reduce heat; cook and stir 2 minutes more. Remove from heat. Stir about 1 cup of the hot filling into the egg yolks. Return to saucepan and bring to a gentle boil. Cook and stir 2 minutes. Remove from the heat and stir in vanilla. Pour hot filling into pie crust. For meringue, immediately beat egg whites with cream of tartar until soft peaks form. Gradually add sugar and continue to beat until stiff and glossy. SPread evenly over hot filling, sealing meringue to pie crust. Bake at 350 degrees for 12 to 15 minutes or until golden.