Butter is essentially a smooth mixture of fat and water. The secret to making beurre blanc is to preserve its makeup by allowing each addition of butter to melt smoothly into the sauce, as you whisk it, before adding the next piece of butter. Never let the sauce come to a boil once the butter is added, which will cause it to separate. Some restaurant cooks add a little heavy cream to the wine reduction before whisking in the butter, to ensure a smooth and stable sauce. This recipe is based on one in Mastering the Art of French Cooking by Julia Child, Louisette Bertholle, and Simone Beck (Knopf, 1977).