- Place foie gras in a glass dish.
- In a small bowl, combine sugar and salt. Sprinkle salt mixture over both sides of each slice. Drizzle with cognac. Cover with plastic wrap and let marinate for about 30 minutes at room temperature or until foie gras is at room temperature and soft.
- Pat slices dry. Place slices in jars, pressing well. Cut slices, if needed. Close jars.
- Off the heat, place jars in a small saucepan. Add enough boiling water to fill up the pan to three quarters of the height of the jars. Cover pan and let stand off the heat for about 40 minutes. The inside of jars needs to reach about 55 °C (131 °F). Remove jars from water and let cool. Refrigerate for at least 24 hours.
- When ready to serve, season with pepper, if desired. Jars will keep for about a week in the refrigerator. Once opened, consume within 48 hours.