Custard

Custard is a variety of curlinary preparations based on a cooked mixture of milk or cream and egg yolk. Depending on how much egg or thickener is used, custard may vary in consistency from a thin pouring sauce (cre'me anglaise) to a thick pastry cream (French: creme patissiere) used to fill éclaise. Most common custards are used as desserts or dessert sauces and typically include sugar and vanilla, sometimes flour, corn starch, or gelatin is added as in pastry cream or crème patissiere. Custard is usually cooked in a double boiler (bain-marie), or heated very gently in a saucepan on a stove, though custard can also be steamed, baked in the oven with or without a water bath, or even cooked in a pressure cooker. Custard preparation is a delicate operation, because a temperature increase of 3-6°C (5-10°F) leads to overcooking and curdling.
 Generally, a fully cooked custard should not exceed 80°C (176°F), it begins setting at 70°C (158°F). A water bath slows heat transfer and makes it easier to remove the custard from the oven before it curdles.
                                                           History
Mixture of milk and eggs thickened by heat have long been part European cuisine, since at least Ancient Rome. Custards baked in pastry (custard tarts) were very popular in the middle Ages, and are the origin of the English word 'custard': the French term 'croustade' originally referred to the crust of a tart, and is derived from the Italian word crostata and ultimately the Latin crustare.Examples include crustardes of flessh  and crustade, in the 14th century English collection The former of cury. These recipes include solide ingredients such as meat, fish and fruit bound by the custard, stirred custards cooked in pots are also found under the name creme Boylede and cream boiled.
 In modern times, the name 'custard' is sometimes applied to starch thickened preparations like blacmange and Birds custard.
                                                                    Custard Variations
While custard may refer to a wide variety of thickened dishes, technically (and French cookery) the word "custard" (crème or more precisely crème moulee) refers only to an egg thickened custard. When starch is added the result is called pastry cream (French:crème patissiere) or confectioners custard, made with a combination of cream or milk, egg yolks, fine sugar, flour or some other starch, and usually a flavoring such as vanilla, chocolate, or lemon. Crème patissiere is a key ingredient  in many French desserts including mille-feuille (or Napoleons) and filled tarts. It is also used in Italian pastry  and sometimes in Boston cream pie. Although egg thickens the custard, the effect is minimal, with the majority of thickening resulting from starch. Corn flour or flour thicken at 100°C and as much many recipes instruct the pastry cream to be boiled.
 In a traditional custard such as crème anglaise, where egg is used alone as a thickener, boiling results in the overcooking  and subsequent 'curdling' of the custard: However, in a pastry cream, starch prevents this. Once cooled the amount of starch in pastry cream 'sets' the cream and requires it to be beaten or whipped before use.