Preheat your oven to 375°F (190°C). Line large baking sheets with parchment paper to prevent the whoopie pies from sticking. Getting the pans ready at the start helps the baking process go smoothly.

Sift the all-purpose flour, Dutch-process cocoa powder, baking soda, and salt into a medium bowl. Set aside. This step ensures your dry ingredients are evenly distributed and removes any lumps, resulting in a lighter, more tender cake.

In the bowl of an electric mixer fitted with the paddle attachment, beat the unsalted butter and dark brown sugar on low speed until combined. Increase speed to medium-high and beat for about 3 minutes, until the mixture is light and fluffy. Scrape down the sides of the bowl. Add the egg and vanilla extract, and beat on medium-high speed for another 3 minutes, or until the mixture is very well combined and creamy.

Add half of the sifted dry ingredient mixture (from Step 2) to the butter mixture (from Step 3), then add half of the buttermilk. Mix on low speed until just combined, scraping the bowl as needed. Add the remaining buttermilk and the rest of the dry ingredients, and continue to mix until just combined. The batter should be fluffy and slightly sticky, resembling a very thick cake batter. I find that alternating between dry and wet helps keep the batter smooth and prevents overmixing.

Use a medium (about 1 ½-tablespoon) spring-loaded scoop to drop mounds of batter onto the prepared baking sheets, leaving about 2 inches between each mound. Bake for 11 minutes, or until the cakes spring back when pressed gently and appear dry on top. Let the cakes cool on the pans for 5 minutes before transferring them to a wire rack to cool completely. I always check the cookies a minute early to make sure they don’t overbake—they should be just set.

In the clean bowl of an electric mixer, beat the marshmallow creme and unsalted butter on low speed until smooth and combined. Scrape down the sides and bottom of the bowl. Add the powdered sugar, vanilla extract, and sea salt. Mix on low until incorporated, then increase to medium-high and beat until the filling is very smooth, about 3 minutes. For a fuller flavor, I sometimes add a pinch of extra salt to the filling.

Use a medium (1 ½-tablespoon) spring-loaded scoop or a spoon to dollop marshmallow filling (from Step 6) onto the flat side of half of the cooled cakes (from Step 5). Top each with another cake to form a sandwich, pressing gently. For neat edges, you can smooth the filling with a small spatula. If transporting or for a fudgier texture, chill the assembled whoopie pies in the fridge for at least an hour before serving.