Beurre blanc sauce
Beurre blanc—literally translated from French as "white butter"—is a rich, hot butter sauce made with a reduction of vinegar or white wine (normally Muscadet) and shallots into which cold, whole butter is blended off the heat to prevent separation. (Lemon juice is sometimes used in place of vinegar and stock can be added as well). This sauce originates in the Loire Valley cuisine.
It is not uncommon to see recipes that include a beurre blanc sauce to which double cream has been added as a "stabilising agent". This is a point of contention amongst many culinary enthusiasts and can be heavily frowned upon.