Whisk together the flour, cocoa, espresso powder, baking soda, and salt in a bowl until combined. Stir together buttermilk and vanilla in a separate bowl.
Beat together softened butter and brown sugar in an electric mixer and medium-high speed until pale and fluffy, 3-5 minutes. Add egg, beating until well combined. Scrape down sides of the bowl. Reduce speed to low and alternately add flour mixture and buttermilk, beginning and ending with flour. Scrape down sides occasionally and mix until just smooth.
Using a cookie scoop (any size you prefer), scoop out mounds of batter 2 inches apart onto 2 large baking sheets that are either buttered or layered with parchment. Bake in the upper and lower thirds of the oven, rotating the sheets halfway through, until the tops are puffed and caked spring back when touched, 11-13 minutes. Transfer to a cooling rack and allow to cool completely.
Make the filling:
Beat together butter, confectioners sugar, marshmallow and vanilla at medium speed until smooth, 3 minutes. Set aside.
Assemble whoopie pies:
Spread filling evenly on flat sides of half the cakes and top with the remaining cakes!
Gourmet claims that these are better the second day. So... try not to eat them all at once.