In a small heavy pan, boil the vinegar, wine and shallots until all liquid is reduced. Add the cream, and re-boil until completely reduced.

Whisk in the cold butter, a few pieces at the time. The sauce thickens and becomes creamy.

Note: do not overheat the sauce, as the sauce separates and loses its creamy texture. Season to taste with salt and pepper.

Keep the sauce warm over a pan of warm water. Serve as soon as possible.