This classical French regional sauce is traditionally served with seafood, poached fish, and delicate green vegetable such as asparagus. It has only four simple ingredients that are apparent: the main ingredient is really time. You need to make this at a time when you really can dedicate yourself to standing at the stove. It's just one of those things in cooking that you can't really cheat at that way. The sauce truly is delicious, and you should enjoy it at least once in a while. Just enjoy it in small quantities, as it does pack a punch in calories: the main ingredient really is butterfat!
Put in a small saucepan the white wine, the vinegar and the shallot. Simmer slowly until it reduces to the consistency of a thin syrup -- about 5 minutes.
Ensure that the heat is very low. Then, start adding the butter a tablespoon at a time. Put the first tablespoon in. Allow it about 1 minute to melt slowly, and when it's almost melted you can stir or whisk the rest in. Repeat for the remaining 15 tablespoons of butter.
You may or may not wish when finished to strain out the shallot; both options are valid. You can season to taste with salt and pepper if desired.
If it seems like a lot of shallot at the start, don't worry -- remember, there's a lot of butter coming.Most people advise to make this on the spot as needed, but that's not practical for home cooks who need to also cook something to serve this sauce on. So you'll want to make this in advance, the same day if possible, earlier that day. Don't put it in the fridge.To keep warm, put in a thermos, or, put in a jar with a lid on and set in a bowl of very hot tap water. The sauce will likely separate on you somewhat. A good stir should bring it back together easily. If it doesn't, then put a tablespoon in another bowl, and whisk it. Add another tablespoon, whisk it in. Repeat until all the sauce has been added into the bowl and whisked.If you need to make it a day ahead or have leftover, store that in a fridge in a sealed jar owing to the presence of the shallot in it. To reheat, reheat slowly by floating the jar (with its lid still on) in a bowl or pan of very hot tap water. Whisk to bring the sauce back together.