1 cup dry white wine
½ cup white wine vinegar
1 Tbsp finely chopped shallot
1 lb unsalted butter, cold
Kosher salt, to taste
Cut the butter into a medium (½-inch) cubes and return the butter cubes to the refrigerator to keep them cold, which is very important.
Heat the wine, vinegar, and shallots in a saucepan until the liquid boils, then lower the heat a bit and continue simmering until the liquid has reduced down to about 2 tablespoons. This should take about 10 minutes.
Once the wine-vinegar mixture has reduced to 2 tablespoons, reduce the heat to low, take the cubes of butter out of the fridge and start adding the cubes, one or two at a time, to the reduction, while you whisk rapidly with a wire whisk.
As the butter melts and incorporates, add more butter and keep whisking. Continue until you only have 2-3 cubes remaining. Remove from heat while whisking in the last few cubes, and whisk for a moment or two more. The finished sauce should be thick and smooth.
Season to taste with Kosher salt. Traditionally the shallots would be strained out before serving, but doing so is optional. Serve right away.