Mix sugar, cocoa, cornstarch and salt together in a pan.
Add in milk. Bring mixture to a boil and boil for 1 minute.
Beat egg yolks together in a bowl.
Add small amount of hot milk mixture into eggs, stirring as you add. You do not want to cook the eggs.
Pour egg/milk mixture into pan with remaining milk mixture.
Over medium heat, stir constantly for about 3 minutes or until thickened.
Remove from heat and add in vanilla, chocolate chips and butter and stir until melted.
Pour mixture into baked pie crust. Add meringue layer while filling is still hot.
Beat 4 egg whites, pinch of salt and cream of tartar until stiff.
Add sugar and few drops of vanilla.
Spread on top of warm pie and bake at 325 degrees F for approximately 20 minutes or until golden brown.
Be sure to seal meringue to edge of pie crust so no air can get in.
Allow pie to cool and set up before serving.
To cut a meringue pie, dip your knife into warm water first. This will prevent the meringue from sticking to the knife.Leftovers will last up to two days in the refrigerator. Place toothpicks into the meringue and gently lay plastic wrap over the top.Be sure that meringue covers the entire surface of the pie and is sealed around the edges to prevent shrinkage and weeping.