This sauce is served over steamed puddings, fruit crisps (crumbles), and any dessert that would be enhanced by a creamy vanilla sauce. It's the perfect homemade replacement for Byrd's Custard powder. One warning, however: 1) use a small, "thick-bottom" saucepan, and 2) cook over low heat and don't allow the mixture to boil or it will curdle! If it does curdle, you might be able to salvage it by blending briefly in a blender. [Measurements are U.S. Standard}
In a small bowl whisk the yolks (or egg) with the sugar to blend.
Whisking all the time, slowly whisk in the scalded milk to prevent egg from curdling until blended. Pour into heavy saucepan.
Over medium-low heat cook the mixture, stirring constantly with a metal spoon. It's essential to stir constantly so it will cook evenly and not curdle. Eventually it will begin to thicken slightly. When the sauce coats the spoon, remove it from the heat. Stir in the vanilla and chill. It will thicken slightly, but will be of pouring consistency like thick cream. If it does boil and curdle, blend briefly in a blender while still hot, then chill it.