I forgot to mention these treats in my chocolate cake and buttercream post... My mom used to do these for us kids. Use your leftover frosting and spread it on graham crackers to make sandwiches! She used to keep them in the fridge, but I didn't.
Tip: I read that you can keep a frosted buttercream cake out for a few days and it'll be ok to eat. Something about there being enough fat in the frosting it won't spoil. I did so with the Dino cake, because it was so big, and it was fine.