Position a rack in the center of the oven and preheat to 375°. Line two baking sheets with parchment paper.In a medium bowl, sift together the flour, cocoa powder, baking soda and salt.In the bowl of a stand mixer fitted with the paddle attachment, beat together the butter, shortening and brown sugar on low speed until just combined. Increase the speed to medium and beat until fluffy and smooth, about 3 minutes.
Add the egg and vanilla and beat for another 2 minutes.Add half of the flour mixture and half of the milk to the batter and beat on low until incorporated. Add the remaining flour mixture and milk and beat until combined.Drop by tablespoons of batter onto the prepared baking sheets at least 2 inches apart.Bake one sheet at a time for about 10 minutes each, or until the rounds spring back when pressed gently. Remove from the oven and let the cakes cool in the pan for 5 minutes before transferring them to a rack to cool completely.To make the filling: In the bowl of a stand mixer fitted with the paddle attachment, beat together the marshmallow fluff and the vegetable shortening, on medium speed until the mixture is smooth and fluffy, about 3 minutes.Add the confectioners’ sugar and the vanilla and beat on low until incorporated. Increase mixer speed to medium and beat until fluffy, about 3 minutes more.Using a knife or spoon, spread the filling onto the flat side of a cooled cake. Top with another cake, flat side down. Repeat with the remaining cakes and serve.