Preheat oven to 350F and line two baking sheets with parchment.

Whisk flour, cocoa powder, baking soda and salt together in a bowl.

Cream butter and sugar until fluffy, then beat in egg and vanilla.

Alternate adding the dry ingredients and buttermilk into the butter mixture, starting and ending with the dry ingredients, mixing just until smooth.

Scoop tablespoon-sized mounds onto the sheets, spaced 2 inches apart, and shape each into a slight oval with the back of a spoon.

Bake 10 to 12 minutes, until the tops spring back when touched lightly.

Cool completely on the sheets, about 15 minutes.

Beat butter, powdered sugar, marshmallow fluff and vanilla together until light and fluffy.

Spread or pipe filling onto the flat side of half the cakes, then sandwich with the remaining cakes.

Pipe thin white icing lines across the top of each sandwich to look like football laces.

Chill 10 minutes to set the filling before serving.