An elegantly simple custard sauce makes a velvety topping for fresh fruit or steamed puddings, turns plain cake into a sumptuous trifle and serves as the golden pond over which poached meringues drift in Floating Island. You can also use the creamy sauce to enhance summer's fresh fruits or to revive drying pieces of cake. Add more milk and lace with spirits, if you like, and you have eggnog. Enrich with cream and your favorite flavoring ingredients to make French-style frozen custard, the richest and smoothest of all ice creams. Nip soft custard with liqueur or Marsala wine and whip it while cooking and you'll produce the frothy delight known as sabayon to the French and zabaglione to the Italians.
Some cooks prefer to prepare soft custard over water in a double boiler for greater control over the coagulation process. It also works to constantly stir while cooking over low heat in a heavy saucepan.
The usual custard proportions are 1 egg plus 2 tablespoons sugar for each cup of milk. This is the minimum ratio of eggs to milk which will produce a properly thickened custard, although as many as 4 eggs may be used and the sugar may be increased to 1/4 cup. Increasing the sugar makes the custard less firm and lengthens the cooking time. Increasing the egg makes the custard more firm and shortens the cooking time. Two egg yolks may be substituted for 1 whole egg. Two egg whites will also thicken the custard as much as 1 whole egg, but the characteristic color and flavor will be missing.
Stir constantly while cooking soft custard and be especially alert after about 12 to 15 minutes. Over low heat, it will take about 15 to 20 minutes for a standard recipe to reach doneness and the last few minutes are crucial. An undercooked custard sauce will be thin and watery; an overcooked sauce will curdle. The difference is a matter of only a few degrees. Test by dipping a metal spoon into the custard, using a thermometer, or both. When done, a thin film should cling to the spoon; the thermometer should register 160 degrees F.
To prevent overcooking and curdling, when it's done, immediately remove the custard from the heat and cool it quickly. If the custard is to be served warm, use it immediately and refrigerate any leftovers. To serve chilled, prevent a skin from forming by pressing plastic wrap onto the surface of the custard. Chill thoroughly.
For safety, always make eggnog and ice cream from a cooked custard base.