Duchess potatoes (french: pommes de terre duchesse) consist of a purée of mashed potato, egg yolk, and butter, which is forced from a piping bag or hand-moulded into various shapes which are then baked in a high temperature oven until golden. they are typically seasoned similarly to mashed potatoes with, for example, salt, pepper, and nutmeg. they are a classic item of french cuisine, and are found in historic french cookbooks.

Source: Wikipedia