1. Combine shallots, vinegar, and tarragon in an 8- to 10-inch frying pan. Boil over high heat, stirring often, until all the liquid evaporates, about 2 minutes.
2. Add 1/2 cup cream, peppercorns, and mustard. Boil, stirring often, until cream turns a golden brown, about 8 minutes. Stir in cognac and remaining cream. (If making ahead, set aside up to 2 hours, or cover and chill up to 1 day.)
3. Heat sauce, stirring, until it boils. Add salt to taste.
Nutritional analysis per tablespoon.