anatomy of a whoopie pie
One part whoopie, aka "sugary icing goodness," and one part pie, aka "cake."
In this instance, the cake is chocolate. This is the standard cake type. Other types include vanilla, vanilla with chocolate chip, and special holiday kinds like gingerbreadish. The icing can also differ, such as the vanilla cake with peanut butter icing (I think that's what it is, I haven't eaten it yet) version from my care package, but plain ol' white is the best.
These little guys are best put in the freezer so the moistness of everything doesn't just make it crumble in your hands when you try to eat it. When I was a kid—and yeah, who are we kidding, I still do it—I would separate the cake from the icing and eat it in thirds: top cake, bottom cake, and then the whole chunk of icing.