Beurre blanc —literally translated from French as "white butter"— is a hot emulsified butter sauce made with a reduction of vinegar and/or white wine (normally Muscadet) and grey shallots into which cold, whole butter is blended off the heat to prevent separation. The small amounts of lecithin and other emulsifiers naturally found in butter are used to form an oil-in-water emulsion. Although similar to hollandaise in concept, it is not considered either a classic leading or compound sauce. This sauce originates in Loire Valley cuisine.
The chef Clémence Lefeuvre (née Clémence Prau) invented beurre blanc, apparently by accident, some time around the beginning of the 20th century. She served this sauce at her restaurant "La Buvette de la Marine" in the hamlet of "La Chebuette" in the village of Saint-Julien-de-Concelles situated on the banks of the Loire River a few kilometers upstream from Nantes. Legend holds that she intended to prepare a béarnaise sauce to go with pike but forgot to add the tarragon and egg yolks. Some sources claim that this invention occurred while she worked as a cook for the Marquis de Goulaine at Château de Goulaine. Aristide Briand, long-time Prime Minister of France and Nobel Peace Prize laureate, said at her death in 1932 that her death "was a bit like national mourning".
A good beurre blanc is rich and buttery, with a neutral flavor that responds well to other seasonings and flavorings, thereby lending itself to the addition of herbs and spices. It should be light and airy yet still liquid, while thick enough to cling to food. Beurre blanc is prepared by reducing wine, shallots, and herbs, if used, until it is nearly dry. Although not necessary, cream can be added at this point to act as a stabilizer to the sauce. Lemon juice is sometimes used in place of vinegar, and stock can be added as well. Cold, one-inch cubes of butter are then gradually incorporated into the sauce as the butter melts and the mixture is whisked. The sauce can separate by either overheating or cooling. If it heats past 136 °F, some of the emulsifying proteins will begin to break down and release the butterfat they hold in emulsion. If the sauce cools below 80 °F, the butterfat will solidify.
Derivatives of beurre blanc sauce 
Beurre Rouge- substitute a dry red wine for the white wine and red wine vinegar for the white wine vinegar.
Lemon-Dill- add heated lemon juice and chopped fresh dill to the sauce.
See also