In a 1 1/2 quart saucepan combine vinegar, wine, shallots, salt and white pepper and bring to a simmer.
Gently simmer liquid until reduced to 1 1/2 tablespoons. Remove from heat and immediately swirl or whisk in 2 pieces of chilled butter. As the butter is incorporated into the liquid add another piece and continue to swirl or whisk. Return pan to low heat and while constantly swirling or whisking add successive pieces of butter. When all of the butter has been added remove from heat. Sauce will be thick and creamy. Adjust seasoning to taste. Serve immediately. To hold the sauce, set the pan in a larger pan of lukewarm water. If the sauce separates, transfer a spoonful of the sauce to a cold mixing bowl and gradually whisk the rest of the sauce in by the spoonful. The sauce can be reheated by gradually whisking 2 to 3 tablespoons of hot liquid, i.e. water, cream or stock.
Recipe courtesy of Gourmet Magazine