Preheat your oven to 350°F (180°C). Sift together the all-purpose flour, unsweetened cocoa powder, baking soda, and salt, then set aside. In the bowl of an electric mixer fitted with the paddle attachment, cream the room-temperature unsalted butter and granulated (or brown) sugar together until the mixture becomes light and fluffy, about 3 minutes. Add the egg and vanilla extract, mixing until combined, then blend in the buttermilk. Gradually add the sifted dry ingredients and mix just until the batter is combined—avoid overmixing for tender cakes. I always recommend scraping down the sides of the bowl to ensure everything is evenly incorporated.

Line two baking pans with parchment paper. Using a medium ice cream scoop or a tablespoon, scoop mounds of batter approximately 1 1/2 tablespoons in size onto the prepared pans, making sure to space them 2–3 inches apart to allow for spreading during baking. Bake the cakes in the preheated oven for 10–12 minutes or until the tops spring back gently when touched. Once baked, carefully transfer the cake rounds to a wire rack and allow them to cool completely before filling.

In a large bowl, combine the room-temperature unsalted butter, marshmallow creme, powdered sugar, vanilla extract, and a pinch of salt. Beat the mixture with a mixer until the filling becomes creamy and fluffy—this may take a few minutes. If the filling is too stiff to spread, you can mix in a little milk or heavy cream, a tablespoon at a time, until you reach your desired consistency. For an extra-fluffy filling, I like to beat it an extra minute for more airiness.

Once the cakes have fully cooled, spread a generous amount of marshmallow filling onto the flat side of half of the cake rounds. Top each with a remaining cake round, flat side in, and gently press so the filling spreads just to the edge. Continue with the rest of the cakes and filling. I sometimes add a little extra filling for extra indulgence—it's the best part!

Place the assembled whoopie pies in the fridge, well covered, for up to a week. For optimal taste and texture, allow them to come to room temperature before serving. Enjoy your freshly baked whoopie pies!