Preheat your oven to 350ºF and grease a baking sheet with cooking spray.
In a small bowl whisk together flour, cocoa powder, baking soda, and salt. Set aside.
In a medium bowl beat together butter and brown sugar with an electric mixer until light and fluffy. Add egg and beat until incorporated. Add flour mixture and buttermilk and vanilla, mixing until smooth.
Spoon 1/4 cup mounds of batter onto the baking sheets, leaving 2 inches between each mound. Bake until tops are puffed and cakes spring back, about 10-12 minutes. Remove from oven and cool completely.
For The Marshmallow Filling
In a large bowl beat together butter, powdered sugar, marshmallow cream, and vanilla until smooth and cream. Spread about a tablespoon of filling on each cake and sandwich with another. Dust with powdered sugar if desired.
Notes
You can prepare these cakes a day in advance and filled the next day.
Make mini pies by using a tablespoon measurement when dishing out the cakes.
Perfect Pairing
Serve these pies as a dessert to New England Clam Chowder.
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