Whisk the egg yolks together in a bowl and set next to the stove. In a medium saucepan, mix together the sugar, cocoa powder, flour and salt. Whisk in the beaten egg yolks and a little of the milk, mixing until smooth. Turn heat to medium and whisk in remaining milk and water. Continue whisking until mixture bubbles and thickens (this took me about 8 minutes since I started with cold ingredients).
When mixture bubbles, continue whisking for another 2 minutes. Remove from heat and whisk in the butter and vanilla.
Pour the mixture into the baked pie crust and quickly make the meringue. You want to spread the meringue on the pie filling while it’s hot.
In bowl of a stand mixer (or regular mixing bowl), beat the egg whites and cream of tartar until soft peaks start to form. Slowly beat in the sugar and continue beating until peaks are stiff. Beat in the vanilla. Spread meringue across the pie.
Bake at 350 degrees F for 10 minutes or until meringue is nicely browned.
Cool at room temperature for about an hour then transfer to refrigerator and chill for a few more hours.