Chocolate meringue pie

1 Pie shell, 9"; baked
1/2 c Cocoa, Dutch process
-unsweetened
1/4 c Cornstarch
1 1/2 c Sugar
1/4 ts Salt
3 Egg; separated
2 c Milk
2 ts Vanilla (not extract)
2 T Butter
3 Egg white
1/4 ts Salt
1 ts Vanilla (not extract)
1/4 ts Cream of tartar
1 ts Baking powder
2/3 c Sugar

Pie: Combine dry ingredients in a medium saucepan and stir to mix thor-
oughly before you add liquids. Now add egg yolks (save the whites for
the meringue) and milk and whisk until there are absolutely no lumps.
Place directly over medium-low heat, stir CONSTANTLY until the pudding
thickens and comes to a plopping boil. Watch it like a hawk and DON'T
TURN UP THE FIRE; at this stage it'll stick and burn at the least
provocation! Patience is essential here. Remove from heat and stir in
the vanilla and butter. Pour into baked pie shell. Top with meringue.
Bake at 325 F. until the meringue browns, 15 to 20 minutes.
Meringue: Beat egg whites and all other ingredients except sugar until
foamy. Add sugar, ONE TABLESPOON AT A TIME, while beating vigorously
until the whites will hold a firm peak and are satiny looking. Spread
meringue over the hot filling, starting with small amounts at the edges
and sealing to the crust all around. If you don't make sure the meringue
is well stuck to the crust at this stage, it'll shrink and pull away
during the baking, which makes a lots less attractive pie. Spread pie
with remaining meringue, making hills and valleys.
Adding the sugar a little at a time helps make sure that it dissolves
completely, and so helps prevent the meringue from weeping. Also,
spreading the meringue over the hot filling helps--I don't know why