Combine the wine and shallots in a small heavy saucepan. Bring to a boil and cook until reduced to 1/3 cup, about 6 minutes. Whisk in the cream, mustard, salt, and cayenne. Boil until thickened by 50 percent, about 5 minutes. Remove from heat, stir in the tarragon, and adjust seasoning, to taste. Serve immediately.
Yield: about 1 1/4 cups