-
1
Fill a medium saucepan with 1 inch of water and bring it to a bare simmer over low heat.
-
2
Meanwhile, combine the vinegar, wine, shallots, tarragon stems, and crushed peppercorns in a medium frying pan.
-
3
Bring to a simmer over medium heat and cook until reduced to just over 2 tablespoons, about 10 minutes.
-
4
Strain the vinegar mixture, pressing on the solids, through a fine-mesh strainer and into a medium heatproof bowl large enough to sit suspended over (but not touching) the water in the saucepan; discard the solids.
-
5
Add the egg yolks to the vinegar mixture and whisk to combine.
-
6
Set the vinegar-yolk mixture over the simmering water and cook, whisking constantly, until the yolks thicken and the mixture forms ribbons when the whisk is lifted from the bowl, about 3 to 4 minutes.
-
7
(Check to make sure the water does not boil by periodically removing the bowl from the saucepan using a pot holder or dry towel.
-
8
If the water boils, reduce the heat so the eggs do not curdle.)
-
9
Whisk in the butter pieces one at a time, making sure each piece is incorporated before adding the next, until all of the butter has been added, about 2 minutes total.
-
10
Remove the sauce from heat.
-
11
(If the sauce is too thick, you can thin it with a few drops of water.)
-
12
Whisk in the chopped tarragon leaves and chervil (if using) and season with salt and pepper.
-
13
Serve warm.
