For the Chocolate Cakes:

Cup of Yum
1. Preheat oven to 350º F. Line 2 baking sheets with Silpats or butter the baking sheets. Set aside.
2. In a medium bowl, whisk together flour, cocoa, baking soda, and salt until combined.
3. In a small bowl, stir together buttermilk and vanilla.
4. In the bowl of a stand mixer, on medium-high speed, beat together butter and brown sugar until pale and fluffy, about 3 minutes. If using a handheld mixer, beat for a couple extra minutes. Then add egg and beat until well combined. Reduce speed to low and alternately mix in flour mixture and buttermilk mixture in batches, beginning and ending with flour, scraping down side of bowl occasionally, and mixing until smooth.
5. On your prepared baking sheets, spoon mounds of batter (equalling a generous 2 tablespoons each – this will give you a nice large pie that isn’t overly huge), leaving close to 2 inches between mounds. Bake in upper and lower thirds of oven, switching position of sheets halfway through baking, until tops are puffed and cakes spring back when touched, about 11 minutes. Remove from oven and let cakes cool on baking sheet for 1 to 2 minutes before transferring with a thin spatula to a rack to cool completely.

For the Fluffy Peanut Butter Marshmallow Cream:

1. In a medium bowl, with a mixer at low speed, combine marshmallow cream, peanut butter, butter, and salt until well blended.
2. On low speed, beat in powdered sugar, alternately with cream, until smooth and fluffy. Beat in vanilla.

To Assemble the Whoopie Pies:

1. Place a tablespoon of peanut butter marshmallow cream onto the center of one of the flat sides of a cake, and then spread the cream almost out the edge.
2. Top with another flat side of cake and press very gently, spreading the frosting out just a bit more to the edge. Repeat with remaining cakes.