In a heavy, nonreactive 2½-quart saucepan, combine the vinegar and sugar. Bring to a boil, stirring to dissolve the sugar. Continue to cook over moderate heat until the vinegar is reduced to a thick syrup.
Swirl in the demi-glace and bring to a boil. Add the cream but do not stir; the cream will be “swallowed up” by the sauce. Boil vigorously for 5 minutes, or unt