You don’t need to use beautiful whole slices of foie gras to make this foam; just the fat will do. Foie gras fat—the fat that is rendered when foie gras is cooked in a terrine or en torchon—is a delicious alternative to butter, duck fat, or goose fat. While normally insoluble in liquid, here Glice (mono- and diglycerides) helps the fat to emulsify.