A cat's tongue is a sweet, crunchy cookie, usually made with egg white, wheat flour, sugar, and butter, and formed in a long flat oval shape something like the tongue of a cat. Many regional variants of the cookie exist, including the langue du chat (French), lingua di gatto (Italian), and lengua de gato (Spanish).
In many regional cuisines, a sandwich cookie is made by putting ganache, chocolate, or ice cream between two cat's tongue cookies.