Whoopie pies are also called Chocolate Gobs, at least here in Pittsburgh. However, only about an hour away, the Amish call them Whoopie Pies. They are 3-4" mounds of chocolate cake (like large soft, puffy cookies) put together in pairs with a fluffy, marshmallowy filling. Never tasted one I didn't like! As previously stated by other posters, there are lots and lots of recipes online.
I've also seen them made from vanilla (yellow)cake, spice cake and pumpkin cake (popular at Halloween, Thanksgiving, etc.)