Chocolate Whoopie Pies

1.Preheat oven to 350°F.
2.Whisk together flour, cocoa, baking soda, and salt in a medium bowl until combined. Stir together buttermilk and 1 teaspoon vanilla in a small bowl.
3.Beat together 1/2 cup butter and brown sugar in a large bowl with an electric mixer at medium-high speed until pale and fluffy, about 5 minutes. Then add egg, beating until combined well. Reduce speed to low and alternately mix in flour mixture and buttermilk in batches, beginning and ending with flour, scraping down side of bowl occasionally, and mixing until smooth.
4.Spoon 1/4-cup mounds of batter about 2 inches apart onto 2 large baking sheets with parchment paper. 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 to 13 minutes. Transfer with a metal spatula to a rack to cool completely.
5.Beat together 1/2 cup butter, powdered sugar, marshmallow, and 1/2 teaspoon vanilla in a bowl with electric mixer at medium speed until smooth, about 3 minutes. Chill filling prior to serving.
6.To assemble pies, spread a rounded tablespoon filling on flat sides of half of cakes and top with remaining cakes.
Note: Cakes should be made a day ahead and kept, layered between sheets of wax paper, in an airtight container at room temperature.