In a medium bowl, sift together the flour and cocoa powder. Stir in the sugar.
Add the mixture to a medium saucepan and blend in evaporated milk and vanilla.
Beat the egg yolks slightly and stir into pan until just combined.
Add the butter. Heat, stirring constantly, until the butter is melted. Pour the filling into an unbaked pie shell.
Bake for 35 to 40 minutes or until the pie looks set when gently shaken.
For the meringue:
In the meantime, combine the egg whites and cream of tartar in a clean, dry metal or glass bowl. Beat at medium speed until the egg yolks form bubbles and look frothy.
Add the sugar, one tablespoon at a time. Beat on medium speed between each addition until the sugar is well incorporated. At this point, when you pull your mixer out of the meringue, the point that forms will fall over on top of itself. This is soft peaks.
Keep beating the eggs whites until they become more dense and shiny. You know your meringue is ready when you pull the mixer out and the point stands up straight. This is stiff peaks.
Spread or pipe the meringue over the top of your pie as soon as it comes out of the oven. Spread the meringue all the way to the edge where the crust and the pie filling meet to seal the meringue.
Return the pie to the oven. Bake until the meringue begins to brown, about 10-15 minutes.