It is called Chantilly cake because it is typically decorated with Chantilly cream, which is a type of whipped cream that is sweetened and flavored with vanilla extract. The name "Chantilly" comes from the town of Chantilly in France, which is known for its production of cream and other dairy products. The term "Chantilly cream" is sometimes also used to refer to whipped cream that has been flavored with other ingredients, such as chocolate or liqueurs.