I have a Betty Crocker's New Picture Cookbook, First Edition, from 1961. Growing up, we called it "sick custard," because it was a special treat my mother would prepare when one was in bed sick. I have tried it out on my wife and her sister; they think getting sick is significantly improved by the presence of custard.
In the Dessert Sauces section, "Betty" has an "Easy Custard Sauce", which is simply the "Rich Custard Filling" thinned with a couple cups of milk to the desired consistency. The "Rich Custard Filling" is like this; the instructions are mine, but follow the same course as the original recipe:
1/2 c sugar
1/3 c flour
1/2 t salt
2 c milk
2 eggs, beaten (or 4 egg yolks)
2 t vanilla
Mix your dry goods in a medium saucepan. Stir in the milk. Over medium heat, vigilantly stir until it boils. Allow it to boil for 1 minute. Hope your pan's big enough that you don't get custard all over the stove. Pull it away from the heat and pour some into the eggs. This'll heat up the eggs gradually, so you don't end up with a custardy egg scramble. Which might be good, but isn't what you're after. Dump the eggy-custardy mixture back into the saucepan with the rest of the custard. Return to the heat and heat it back up to a boil. Immediately remove from the heat and mix in the vanilla. Thin, if you feel the urge. Feast. Best performed alone, or in the company of one other favorite person - more folks mean less custard for the cook.
-jbn