Whisk together flour, cocoa, baking soda, and salt in a bowl until combined. Stir together buttermilk and vanilla in a small bowl.
Beat together butter and brown sugar in a large bowl with an electric mixer at medium-high speed until pale and fluffy, about 3 minutes in a standing mixer or 5 minutes with a handheld, 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.
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.
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.
Cooks' notes:• Cakes can be made 3 days ahead and kept, layered between sheets of wax paper, in an airtight container at room temperature.
• Filling can be made 4 hours ahead and kept, covered, at room temperature.
Im just a girl who grew up to be a wife and mother, who now likes to make people happy with fun and pretty food..so this is where the fun begins..I love my life, I love my family, I love my friends and I love that I know who I am and where I came from..I know this story will end Happily Ever After and that's all Im asking for..