CAKES
2 cups all-purpose flour
½ cup Dutch-processed cocoa powder
1 teaspoon baking soda
½ teaspoon salt
1 cup packed light brown sugar
8 tablespoons (1 stick) unsalted butter, softened but still cool
1 large egg, room temperature
1 teaspoon vanilla extract
1 cup buttermilk

FILLING
12 tablespoons (1½ sticks) unsalted butter, softened but still cool
1¼ cups confectioners’ sugar
1½ teaspoons vanilla extract
⅛ teaspoon salt
2½ cups Marshmallow Fluff