In a saucepot, whisk together the cocoa, sugar, flour, salt, egg yolks, and milk.
On medium heat, cook while stirring until it bubbles and thickens, about 5 to 10 minutes. If it becomes lumpy, just beat out the lumps. (It will not get any thicker in the oven so cook until it’s as thick as you want it.)
Remove the chocolate filling from the heat and stir in the vanilla and butter.
Meanwhile, as you make the custard, poke holes in the piecrust with a fork and bake it until it’s brown, about 20 minutes.
To make the meringue, beat the egg whites with salt and when they start to get fluffy add the sugar.
Pour the chocolate custard into the baked pie shell and top with the beaten egg whites. Bake it until it the peaks on the meringue are lightly browned, about 10 minutes. Serve warm.
Grandma says: “It’s real good hot, wonderful cold and you can even eat it frozen—then it’s like a popsicle!”
All that’s happening in the oven is the browning of the meringue. So be sure and keep cooking the custard in the pan until it’s your desired consistency.