4 medium shallots, peeled and finely chopped
1/2 cup dry white wine
1/2 cup white-wine vinegar
4 tablespoons heavy cream
1 pound butter
Salt and pepper, to taste
In a heavy-bottomed saucepan, combine the shallots with the white wine and vinegar. Gently simmer mixture until practically all liquid has evaporated (reduce by about 90 percent).
Add the heavy cream and gently heat to almost a simmer.
Cut the butter into 1-inch cubes and add them to shallot-cream reduction. Whisk sauce over high heat (but don't let liquid boil) until all butter has been incorporated.
Adjust seasoning, adding salt and pepper to taste. If sauce seems flat, add wine vinegar, a few drops at a time. If sauce tastes harsh or overly acidic, whisk in more butter.
If held properly, beurre blanc prepared just before a lengthy meal will stay intact for several hours; leave it in the saucepan, covered, in a warm area, such as a warm oven, plate warmer or on the back of the stove over very low heat. If necessary, saucepan can be placed in a pan of very hot (but not boiling) water. If held for any length of time, it will begin to thicken and must be thinned periodically with heavy cream, water or other appropriate liquid. If it isn't thinned and stirred every 30 minutes or so, it is likely to break.
— Recipe adapted from "Sauces: Classical and Contemporary Sauce Making," by James Peterson.