Makes 12 
- 2 cups all-purpose flour
- ½ cup unsweetened cocoa powder
- 1 ¼ teaspoons baking soda
- ½ teaspoon salt
- 1 cup packed brown sugar
- ½ cup unsalted butter, softened
- 1 large egg
- 1 cup buttermilk, room temperature
- 1 teaspoon vanilla extract
- Filling (recipe to follow)
Filling
- ½ cup unsalted butter, softened
- 1 (7-ounce) jar marshmallow fluff
- ½ cup powdered sugar
- 1 teaspoon vanilla extract
- 1 teaspoon corn starch
- ½ cup heavy whipping cream
Preheat oven to 350 degrees F.
Line 2 baking sheets with parchment. In a bowl, whisk flour, cocoa, baking soda, and salt. In another bowl, beat together the brown sugar and butter until fluffy, add the egg, beat until smooth, then whisk in the buttermilk and vanilla. Add the dry ingredients, mixing until just combined, avoiding overmixing.
Drop the batter by rounded scoop onto baking sheets with 2-inch spacing. Bake 10 to 12 minutes, until cakes spring back when touched. Cool completely.
In a clean bowl, prepare the filling by beating together the butter and marshmallow fluff until smooth and fluffy. Add the powdered sugar, vanilla, and corn starch. In a separate bowl, whip the heavy cream to medium peaks. Gently fold the whipped cream into the marshmallow mixture until light and fluffy.
Assemble the pies by piping the filling onto the flat side of 1 cake and top with another cake, flat side down. Press gently to spread filling to edges. Best stored and served chilled.
Can be frozen for up to 1 month or refrigerated for 2 to 3 days. Will hold best if frozen prior to packing for a picnic.
