For the Cakes: 

1. Adjust oven racks to upper-middle and lower-middle positions and preheat oven to 350℉.
2. Line 2 baking sheets with parchment paper.
3. Whisk flour, cocoa, baking soda, and salt together in a bowl. 
4. Using a stand mixer with a paddle, beat butter and sugar about 3 minutes until fluffy.
5. Add egg and beat until incorporated.
6. Add vanilla and mix on medium speed, scraping sides of the bowl as needed.
7. Reduce speed to low and add flour in 3 additions, alternating with buttermilk in 2 additions. Stir batter by hand to ensure no flour pockets remain. 
8. Using a ⅓-cup measuring cup, scoop 6 mounds of batter onto each baking sheet (about 3 inches apart), making 12 mounds total. Bake 15 to 18 minutes, rotating sheets halfway through baking, until centers spring back when lightly pressed.
9. Let cakes cool completely on sheets. 

For the Filling: 

1. Using a stand mixer with a paddle, beat butter and sugar for about 3 minutes, until fluffy. Add vanilla and salt and mix. Add marshmallow creme and mix for about 2 minutes until combined. Refrigerate for 30 minutes. 
2. Place ⅓ cup of filling on the bottom half of cakes, then top with remaining cakes. Press together to spread the filling to the edge. Serve.