|Sherry vinegar/White wine vinegar||1⁄2 Cup (8 tbs)|
|Dry white vermouth||1⁄4 Cup (4 tbs)|
|Finely chopped shallot||1 Tablespoon|
|Dried tarragon||1⁄2 Teaspoon|
|Sweet butter||1⁄2 Pound, melted (About 2 Sticks)|
Combine the vinegar, vermouth, shallot, tarragon and salt in a small heavy saucepan.
Bring to a boil, lower the heat, and simmer until reduced to a few spoonfuls.
Cool to room temperature.
Strain the cooled mixture into another small heavy saucepan, or the top pan of a double boiler, and whisk in the egg yolks.
Beat until thick and creamy.
Set the pan over very low heat, or over simmering water in the lower pan of the double boiler, and begin whisking immediately.
Continue to whisk until the egg mixture just begins to thicken; the surest sign of this is that the wires of the whisk will begin to leave "tracks" in which you can see the bottom of the pan.
Remove pan from the heat and begin to dribble in the melted butter, whisking constantly.
Incorporate all the butter, but leave the milky residue behind.
Taste, and correct seasoning; add a few more drops of vinegar if you like.
Set sauce aside, covered, in a warm (not hot) place.
The sauce will keep for at least 30 minutes