Melt 2 tablespoons of the butter in a large skillet. Add the chicken, season with salt and pepper and cook over moderate heat until slightly browned, about 4 minutes per side. Add the shallot and cook, stirring, for 1 minute. Add the Cognac and carefully ignite it with a long match. When the flames subside, add the Riesling, cover and simmer over low heat until the chicken breasts are just cooked, about 25 minutes. Transfer the breasts to a large plate and cover with foil. Cover and simmer the legs until cooked through, about 10 minutes longer. Transfer to the plate.