Mom's Old fashioned chocolate pie

Mom's Old fashioned chocolate pie
1-1/4 cup of sugar
1/2 cup of plain flour
1/4 cup of cocoa
dash of salt
4 egg yolks
2 cups of milk
1/4 cup of butter
1 tsp of vanilla
1-9" pastry shell baked

Bake pie crust until golden
Combine first 4 ingredients in a sauce pan; set aside.
Combine milk and egg yolks.
Stir milk and egg yolks into mixture; add butter.
Cook over medium heat, and stir constantly until mix thickens and boils.
Remove from heat; stir in vanilla; spoon into pastry shell.
You can put a meringue on top if you want, but this is only optional using the egg whites from eggs used for pie