Melt butter in a glass measure until completely melted and clear, but not bubbling. Skim off any foam and cool until lukewarm.

Combine vinegar, water, and seasonings in a saucepan and simmer until the liquid reduces to 1 teaspoon.

Mix in 1/2 tablespoon cool water, strain the liquid, and return it to the saucepan.

Whisk in the yolk, then 1/4 of the clarified butter.

Place over medium-low heat and continue whisking until the mixture thickens to a sour cream consistency.

Dunk the pan briefly in cold water; then slowly dribble in the rest of the butter off heat, whisking constantly, without including the milky liquid at the bottom.

When all the butter is absorbed, the sauce should be the consistency of a medium-thick mayonnaise.

Whisk in the cream (or water) so it forms soft, slowly dissolving peaks.

To keep the sauce from congealing, set it in a pan of hot tap water. Serve immediately.

If the sauce overheats or the butter is added too fast, and it separates, put a teaspoon of water in a small bowl, add a spoonful of the separating sauce, and whisk them together until creamy; then gradually add the rest of the sauce, spoonful by spoonful, until the whole thing is reconstituted.