Nantais Butter Sauce

A good sauce for all poached fish. In the region of its origin it classically accompanies poached northern pike and is made with Breton salt butter (unsalted butter of the best quality is always superior to salted butter in France and I have never made it with salted butter—were one to use it, the salt should be eliminated from the recipe). If using the recommended reduced herb vinegar, add a little more white wine and a bit less vinegar. The color of