Prepare foie gras (see Preparing a Foie Gras Terrine), stopping at step 4.
Place foie gras in a medium bowl, break into even pieces, and add sauternes. Season with salt and pepper and allow to marinate 2 hours.
Preheat oven to 200°. Remove foie gras from marinade; press into a 2 1⁄2-cup terrine, leaving a bit of space at top. Place terrine on 3 folded-over paper towels in the bottom of a deep skillet, and fill skillet with hot water to reach halfway up sides of terrine. Cook until internal temperature of foie gras reaches 115° on a meat thermometer, about 30 minutes. Pour off fat and reserve. Cool terrine.
Cut a piece of cardboard to fit inside top of terrine and wrap it in plastic wrap. Gently press cardboard onto foie gras; weight with a small can for 1 hour. Remove can and cardboard, return reserved fat to terrine, cover, and refrigerate 1–2 days.
To unmold, dip terrine in a bowl of warm water for 30 seconds, run a knife along edges, and invert onto a plate. (Reserve fat in terrine.) Serve thinly sliced, garnished with truffle, if desired. If covered in reserved fat leftovers will keep, refrigerated, for 1 week.