Melt the sugar in a 4-quart (4liter) heavy-bottomed saucepan or copper poêlon. (A large saucepan or poêlon is necessary because the cream boils up when added to the caramel.) (A)
Stir the sugar constantly over medium heat with a wooden spoon until any lumps have melted. (B)
Continue stirring until the caramel is a deep reddish brown. (C)
Standing back from the saucepan, add half the cream. The cream will boil vigorously and dissolve the hot caramel. When the boiling slows down, add the rest of the cream. (D)
Whisk the sauce until the caramel is thoroughly dissolved in the cream. Check the consistency of the sauce and reduce it slightly if necessary. Add the vanilla extract or other flavoring if desired. Let the sauce cool. It can be stored for weeks in the refrigerator.