Mix together sugar, cocoa, corn starch and salt in a medium saucepan. Gradually mix in milk. Cook and stir over medium high heat until thickened and bubbly. Reduce heat to medium low; cook and stir 2 minutes more. Remove pan from heat. Stir about one cup of the hot filling into the egg yolks; mix back into the custard. Return saucepan to heat, and bring to a gentle boil. Cook and stir for 2 minutes. Remove from the heat, and stir in vanilla. Pour hot filing into crust.
In a clean bowl, beat egg whites with cream of tartar until soft peaks form. Gradually beat in sugar, and continue to beat until stiff and glossy. Spread evenly over hot filling, sealing meringue to crust.
Bake at 350 degrees F (175 degrees C) for 12 to 15 minutes, or until golden.