Preheat oven to 350 degrees.
Mix together 1 1/2 c sugar, cocoa, flour, milk, and egg yolks.
Cook in double boiler until creamy, stirring constantly.
Pour into pre-baked pie shell.
To make meringue, beat egg whites and cream of tartar slowly with electric mixer.
Slowly add in sugar and vanilla.
Beat until fluffy and smooth.
Spread over pie.
Bake for 3 to 5 minutes or until meringue is lightly browned.