8 tablespoons (1 stick) cold butter, cut into bits
Combine the shallot, vinegar, and wine in a small
saucepan over medium heat. Sprinkle with salt and pepper
and cook, stirring occasionally, until reduced to a couple
of tablespoons, about 5 minutes. Cool for 2 minutes.
Turn the heat as low as possible and whisk in the
butter a bit at a time. As each piece is incorporated, add
the next. When the sauce is creamy and smooth, and all
the butter is incorporated, you’re done. Serve immediately;
this sauce will not keep.