In a medium bowl, add sugar and sift together the cocoa and cornstarch.

Whisk together egg yolks in a small bowl. Heat milk to scalding, just before it begins to boil. Remove milk from heat and slowly add milk to egg yolks while whisking together.

Add egg mixture and dry ingredients together in a medium pot over medium-high heat. Whisk constantly until mixture begins to thicken, about 5 minutes. Once it starts to thicken, stir with spoon until it becomes the consistency of pudding. Remove from heat and stir in butter and vanilla. Let mixture cool slightly before adding to a prepared pie crust.

To make the meringue, whisk egg whites and cream of tartar on high speed until stiff peaks form. Spread meringue over pie filling and bake at 375 for about 15-20 minutes until meringue is browned.