Preheat oven to 375°F. Line large baking sheets with parchment paper.
Sift the flour, cocoa powder, baking soda, and salt into a medium bowl.
In the bowl of an electric mixer fitted with the paddle attachment, beat the butter and brown sugar on low speed until combined. Increase speed to medium high and beat for 3 minutes, or until light and fluffy. Scrape down the sides of the bowl. Add the egg and vanilla and beat on medium-high speed for another 3 minutes, or until very well combined.
Add half of the flour mixture then half of the buttermilk to the batter and beat on low until combined, scraping down the sides and bottom of the bowl as necessary. Add remaining buttermilk and flour, beating until combined. Mixture will be fluffy and slightly sticky like a very thick cake batter.
Use a medium 1 ½-tablespoon spring-loaded scoop to drop mounds of batter onto prepared baking sheets, spacing 2 inches apart. Bake for 11 minutes, or until pies spring back slightly when pressed gently and appear dry on top. Let cakes cool on pans for 5 minutes before moving to a wire rack to cool completely.
For the filling:
In the bowl of an electric mixer, beat the marshmallow creme and butter on low speed until combined. Scrape the sides and bottom of the bowl. Mix in the powdered sugar, vanilla, and salt on low speed until combined then increase to medium high and beat until very smooth, about 3 minutes.
Use a medium 1 ½-tablespoon spring-loaded scoop to dollop marshmallow filling on the flat side of half the whoopie pies. Sandwich gently with the remaining whoopie pies.
If transporting, chill the whoopie pies in the fridge for at least an hour before transporting to make firmer and less fragile. Serve chilled for a more fudgy texture. Store at room temperature in an airtight container for up to a day or in the fridge for up to 4 days.
Whoopie Pies https://handletheheat.com/classic-whoopie-pies/ March 1, 2023