Preheat the oven to 350 degrees. Line 2 baking sheets with parchment paper and set aside.
In a medium mixing bowl, whisk together the flour, cocoa powder, baking soda and salt. Set aside.
In a large mixing bowl, use an electric mixer to combine the brown sugar and butter and beat until light and fluffy. Add the egg and mix just until incorporated. With the mixer on low, add the vanilla and blend until incorporated. Scrape the bottom and sides of the mixing bowl with a rubber spatula and blend another 20 to 30 seconds.
In a 2 cup measure or small bowl, mix together the buttermilk and espresso powder.
With the mixer set on low, add one-third of the flour mixture, then pour in half of the buttermilk mixture. Repeat adding one-third of the flour, then the remaining buttermilk. Finally mix in the remaining flour, scrape down the sides and blend another minute.
Scoop the batter into a 1/3-cup measure and place in a mound on the parchment lined baking sheet. Repeat adding 5 more mounds to the first pan (for a total of 6 per pan) ensuring they are spaced about 3-inches apart. Bake for 16 minutes, rotating the pan once during baking, or until the cakes spring back when lightly pressed. While the first pan is baking, prepare the second pan by mounding the parchment lined baking sheet with six more 1/3 cup scoops of batter. Bake the second sheet then cool all 12 cakes on the parchment for at least 1 hour.
MAKE THE FILLING:
In a medium mixing bowl, use an electric mixer to combine the butter and powdered sugar; beat until fluffy.
Add the vanilla and salt and blend. Beat in the Marshmallow Fluff/Creme until incorporated, about 2 minutes. Scrape the frosting into a large ziplock or piping bag and refrigerate until firm, 45 to 60 minutes.
Cut a 1/2-inch hole at the tip of the bag, and pipe the frosting evenly in the center of the flat side of 6 cakes. Top with the flat side of the remaining 6 cakes and gently press until the filling spreads to the edges of the cake. Serve immediately or refrigerate in an airtight container up to 3 days.