In a saucepan over medium heat, sift together sugar, cocoa, and flour. Whisk in milk and egg yolks. Continue to cook, stirring often until mixture is very thick ( like puddinabout 20 minutes.
Remove from the heat and stir in butter and vanilla. Pour the custard into the pie crust and cool completely.
To make the meringue; Preheat oven to 35In a large, dry bowl whisk egg whites until they are really fluffy.
Sprinkle in the sugar a little at a time and whisk until the sugar is well incorporated and meringue stands in peaks and is shiny.
Pour the meringue on top of the chocolate filling ( make sure it is completely cool or it will melt!). Gently spread the meringue over the pie making sure to spread all the way to the crust. Bake in hot oven until the top is lightly browned, 4-5 minutes. Chill completely before slicing and serving.