For an ah-maze-ing twist on a classic whoopie pie, try filling it with peanut butter frosting. Nom.
Bake the cookie/cake as normal (I suggest using Redbook's recipe and make them all chocolate). To make the frosting, mix 1 cup creamy peanut butter, 1 cup powdered sugar, 1/2 stick (1/4 cup) softened butter, and approx 1 Tbsp milk to a thick frosting consistency. Spread between two cookies/cakes to make your whoopie pie, and enjoy!
BTW - I cooked them two different ways - on parchment paper as well as in a muffin top pan. Cooking them on parchment paper resulted in a firmer cookie that didn't fall apart, though slightly less perfectly round than the pan yielded.
While eating one, my friend might have proposed to me, but it was hard to tell with that much cookie in his mouth and all the happy noises and such. They're that good.