1For Pie Filling:
In a medium sized saucepan combine the sugar, cocoa powder, corn starch, salt, milk and egg yolks. BEFORE turning on the burner, whisk all the ingredients together until well mixed. The corn starch will not dissolve in hot liquids so it is VERY important to stir until it has all dissolved before turning on the burner.
2Cook mixture on medium to medium high heat whisking constantly until pudding starts to thicken.
Remove from heat.
3Stir in butter and vanilla.
4Pour into prepared pie crust.
In a clean mixer bowl, add the egg whites and vanilla. On high speed, whip until whites start to to whip up. Slowly and a little at a time, add the powdered sugar and cream of tartar. Continue whipping until stiff peaks form.
6Spoon meringue on top of the pie filling.
Bake at 325 degrees until meringue reaches the desired browness.
Make sure your mixing bowl is completely clean and free of oil. Meringue will not whip if there is oil residue in the bowl.
To keep meringue from shrinking, make sure it touches all around the edge of the pie crust.