In a saucepan, combine the vinegar, wine, peppercorns, shallots, and tarragon. Bring the liquid to a boil and reduce to 1 tablespoon. Add 1 tablespoon of water. Add the egg yolks and whisk, over low heat, until frothy, about 3 to 4 minutes. In a steady stream, add the butter until the sauce thickens. Season with salt and pepper. Strain the sauce through a chinois and set aside. Garnish with the parsley leaves.