1. Preheat oven to 350°F.
2. Sift together the sugar, cocoa, salt and flour in mixing bowl and set aside.
3. Separate all four eggs. Set egg whites aside in a large bowl for the meringue (see Notes, below). Put the yolks in a heavy saucepan and beat well with a fork or whisk. Then stir in the sifted dry ingredients alternately with the milk, adding just a little at a time to keep mixture smooth.
4. Cook mixture over medium heat, and add vanilla when it begins to boil. Boil, stirring constantly, until mixture thickens. Remove from heat; beat out any lumps that may have formed.
5. Pour into baked pie shell and top with meringue while filling is still hot. Bake in 350°F oven until meringue is lightly browned, 12 to 15 minutes.

Note: To avoid a "weeping" meringue, do not allow the filling to cool before putting on the meringue. We talk in depth about the meringue portion of this recipe in our post Secrets to Good Meringue.

Although this recipe calls for whole milk, I have used 2-percent milk with very good results.