Beat the egg yolks until pale but not dry. Add the sugar, salt and milk. Cook and stir in the top part of a double boiler over simmering water. The water must never bubble into a boil.
When the custard reaches 175°F. on a candy thermometer, it is done and must be removed from the heat, flavored, and put aside to use. If you do not own a candy thermometer, watch the stirring spoon. When it coats all over with the egg mixture and nothing drips from it, 175°F. has been reached.