A very thin, crispy cookie is called a wafer. Chocolate cream sandwiched with wafers is a delicious treat.
In addition to a cookie, wafer can also refer to the thin bread used during the Christian ritual of Holy Communion. These wafers are small and round. The word is used for other thin, disc-shaped objects as well, like an electronic wafer, a circular sliver of material that helps form a circuit. But the most common meaning is still the original "thin cake of paste," from a root that wafer shares with waffle.