Adjust oven racks to upper-middle and lower-middle positions and preheat oven to 350℉.
Line 2 baking sheets with parchment paper.
Whisk flour, cocoa, baking soda, and salt together in a bowl.
Using a stand mixer with a paddle, beat butter and sugar about 3 minutes until fluffy.
Add egg and beat until incorporated.
Add vanilla and mix on medium speed, scraping sides of the bowl as needed.
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.
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.
Let cakes cool completely on sheets.
For the Filling:
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.
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.
Notes
You can make around 24 whoopie pies that are the size of a traditional cookie by placing 1 rounded tablespoon of batter on your prepared sheets and baking the cookies for around 10 minutes. These are giant!
For perfectly textured cookies, make sure all your ingredients are at room temperature. This allows the butter to cream properly, leading to fluffy cakes.
The marshmallow creme filling can get soft at room temperature. To make it easier to spread and prevent it from squishing out the sides, chill the filling for at least 30 minutes before assembling your Whoopie Pies.
For an extra fancy touch, consider using a piping bag to spread the filling. This will give you clean, even lines of filling and a professional look.