Beurre blanc
Beurre blanc or Beurre Nantais ( French pronunciation: [bœʁ nɑ̃tɛ]) is a warm emulsified butter sauce made with a reduction of vinegar and/or white wine (normally Muscadet) and shallots into which softened whole butter is whisked in off the heat to prevent separation. The small amount of emulsifiers naturally found in butter, including a trace of lecithin, is used to form an oil-in-water emulsion. Although similar to hollandaise in concept, it is considered neither a classic leading nor compound sauce. This sauce originates in the cuisine of the Loire-Atlantique department. The chef Clémence Lefeuvre (née Clémence Prau) invented beurre blanc, apparently by accident, sometime 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 on the banks of the Loire River a few kilometers upstream from Nantes.
Source: Wikipedia