The whoopie pie, alternatively called a black moon, gob (a term indigenous to the Pittsburgh region), black-and-white, bob, or "BFO" for Big Fat Oreo (also recorded as "Devil Dogs" and "Twins" in 1835) is an American baked product that may be considered either a cookie, pie, sandwich, or cake. It is made of two round mound-shaped pieces of usually chocolate cake, or sometimes pumpkin, gingerbread, or other flavored cakes, with a sweet, creamy filling or frosting sandwiched between them.

Source: Wikipedia