Preheat oven to 375 degrees. Line two large baking sheets with parchment paper.
Sift flour, cocoa powder, espresso powder, baking soda, and salt together into a medium bowl.
In a large mixing bowl beat butter and sugar on medium speed for 4 minutes.
Add egg and beat for an additional minute, then scrape sides. Add vanilla extract and beat until mixed in.
Add ⅓ of the flour mixture into the batter and mix on low speed until just combined.Add ½ of the buttermilk and mix fully.Add ½ of the remaining flour mix, then remaining buttermilk, and mix until incorporated.Add the rest of the flour mixture and mix in by hand until flour is no longer visible.
Spoon in 2 tablespoons onto baking sheets, leaving plenty of room between them as they will spread.
Bake for 8-10 minutes, rotating the pans halfway through. Press your finger gently into the top of one of the cakes. If it springs back then it is done. If it remains indented, continue cooking.Once baked, remove from oven and cool completely.If you have remaining batter, allow the first batch to cool for 5 minutes, then slide them off of the pan and add a new parchment sheet to cook the remaining cakes.
Beat butter, sugar, and vanilla on medium speed until combined. Increase speed to high and continue beating for 4 minutes.
Gently fold marshmallow cream into the frosting using a spatula.
Place in the refrigerator for 20-30 minutes to thicken.
Fill a pastry bag with the filling, using an open tip. Pipe filling over the flat surface of half of the cookies. Match cookies according to size and place the second cookie over the top.
Store in an airtight container in the refrigerator. Remove to room temperature 30 minutes prior to serving.
When measuring the flour either use a scale to weigh the grams or spoon the flour into a measuring cup and then level the cup. This will help you to measure accurately so you do not end up with crumbly cakes.The nutritional facts provided are only estimates. The accuracy of the nutritional information for any recipe on this site is not guaranteed.