This sauce, although not a foundation sauce, is so very popular in France, and somehow seems so closely associated with French cooking, with their unique tournedos, and other luscious dishes, that I have selected it from countless others. It is said to be the creation of the chef des cuisines of the Pavilion Henri IV at Saint-Germain-en-Laye, and dates from about 1830.
Put the wine and vinegar in a saucepan with the chopped shallot, the tarragon, the chervil, season with the mignonette pepper and a little salt. Bring to the boil and simmer till reduced by two-thirds. Remove from the fire and, when lukewarm, add the 3 yolks of eggs, stirring well, replace on a very slow fire—or even keep the saucepan in another saucepan of boiling water—and very gradually add the butter, divided in small pieces, beating with an egg-whisk incessantly. When all the butter has been used, the sauce should be very light. It should be hurriedly strained through a conical sieve, replaced on the fire with a little chopped tarragon and chervil, and seasoned with a dash of cayenne. This sauce should never be allowed to get really hot. It should be treated as a “butter mayonnaise,” worked as such, and served lukewarm, otherwise it will curdle.