1-1/4 teaspoons baking soda
1 teaspoon salt
1 cup well-shaken buttermilk
1 teaspoon vanilla
1 stick butter, softened
1 cup packed brown sugar
1 large egg
Preheat oven to 350°F. Whisk together flour, cocoa, baking soda, and salt in a bowl until combined. Stir together buttermilk and vanilla in a small bowl.
Spoon 1/4-cup mounds of batter about 2 inches apart onto 2 buttered large baking sheets. 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, 11 to 13 minutes. Transfer with a metal spatula to a rack to cool completely.
1 stick (1/2 cup) unsalted butter, softened
1 1/4 cups confectioners’ sugar
2 cups marshmallow cream such as Marshmallow Fluff
Beat together butter, confectioners’ sugar, marshmallow, and vanilla in a bowl with electric mixer at medium speed until smooth, about 3 minutes. Spread a rounded tablespoon filling on flat sides of half of cakes and top with remaining cakes.