Basic vanilla custard sauce is a mixture of eggs, sugar, milk, salt and flavoring cooked in a double boiler until thickened but still pourable. It's also referred to as stirred custard, boiled custard, soft custard, light custard or creme anglaise.
This type of custard must be cooked slowly over simmering water, stirring constantly. Don't try to hurry the cooking by raising the temperature--intense heat will cause the eggs to curdle. Prepared properly, however, the result is a creamy sauce, slightly thickened by the eggs.
The water in the double boiler should be kept just simmering and should not touch the bottom of the insert. Using a wire whisk, blend the egg yolks well in the top portion of the pan, then whisk in the sugar (Step 1).
Stir in the milk (Step 2) and add a dash of salt, then cook over simmering water, stirring constantly, until the custard reaches 185 degrees or coats the back of a metal or wooden spoon. A finger drawn across the back of the spoon should leave a clean path (Step 3), and the sauce's consistency should be smooth and creamy.