Chop the shallots until they are almost a purée.

Put shallots in a small saucepan with the wine and vinegar and cook until the shallots are completely soft and the liquid all but dried up.

When this mixture has cooled, start adding the butter, about 1 oz. at a time. Keep over a very low flame and whisk for a few seconds. As soon as the butter looks like getting soft and melting, remove the pan from the fire, because at no time during the cooking must the butter completely melt. It must finish up the color and the consistency of thick cream. It only takes a minute or so. Do not let the sauce get too hot.