For a colorful dessert, make three quarts, usingpurple, red, and green plums.
Cut plums in half and remove pits. Cut halves into 1 1/2-inch pieces. Combine plums, 3/4 cup water, the sugar, and the salt in a 5-quart nonreactive saucepan. Cook over medium heat until plums break down, about30 minutes. Stir occasionally to prevent plums from scorching.
Remove from heat and cool slightly. Puree mixture in a food processor and strain into a medium bowl set over an ice bath to chill. Stir in cognac and vinegar.
Freeze mixture in an ice-cream maker according to manufacturer's instructions.