This recipe is found in the cookbook On Cooking by Sarah R. Labensky and
Alan M. Hause. It is listed on page 217

Beurre Blanc

White Wine 1 oz.
White Wine Vinegar 4 oz.
Salt 2 1/2 tsp.
White Pepper 1/2 tsp.
shallots minced 3 tbsp.
whole butter chilled 2lbs

Combine the white wine, white wine vinegar, salt, white pepper and shallots
in a small saucepan. Reduce the mixture until approximately 2 tablespoons
of liquid remain. If more than 2 tablespoons of liquid are allowed to remain the resulting sauce will be too thin. For a thicker sauce reduce the mixture au sec.

Cut the butter into pieces approximately 1 ounce in weight. Over low heat
whisk in the butter a few pieces at a time using the chilled butter to keep
the sauce between 100 and 120 degrees F.

Once all the butter has been incorporated, remove the saucepan from the
heat. Strain through a chinois and hold the sauce between 100 and 130 for
use.

Additional Tips
Use a nonaluminum pan to prevent discoloring the sauce. Over medium heat
reduce the wine shallots and herbs until au sec or almost dry. Some chefs
add a small amount of heavy cream at this point and reduce the mixture.
The added cream helps stablize the finished sauce. Butter should be cold
and added a little at a time. When all the butter is incorporated the sauce
should be strained.