Cut the Foie Gras crosswise into 1/2-inch-thick pieces, then season with salt and pepper.
Heat 1 teaspoon of the butter in a 10-inch heavy skillet over moderately high heat until hot but not smoking.
Sauté half the Foie Gras until golden, 45 to 60 seconds on each side (it will be pink inside). Quickly transfer to a paper towel to drain and discard fat in skillet.
Sauté the rest of the Foie Gras the same way, then discard all all but 1 tablespoon of remaining fat in skillet.
Add 2 tablespoons balsamic vinegar and bring to a boil.
Serve Foie Gras with sauce.
Makes 4 servings