Preheat your oven to 350°F (175°C). Line two large baking sheets with parchment paper or silicone baking mats to prevent sticking and help the cookies bake evenly.

In a medium bowl, whisk together the all-purpose flour, unsweetened cocoa powder, baking soda, baking powder, and fine sea salt until well combined and free of lumps. Set aside.

In a large mixing bowl or stand mixer, beat the softened butter, neutral oil, packed light brown sugar, and granulated sugar on medium speed for 2–3 minutes, until light and fluffy. Scrape down the sides of the bowl as needed.

Beat in the eggs one at a time, mixing well after each addition. Add the vanilla extract and mix until combined. The batter will look looser at this stage.

With the mixer on low speed, add half of the dry ingredient mixture, then half of the buttermilk. Repeat with the remaining dry mixture and buttermilk, mixing just until combined. The batter should be thick and scoopable, like a soft cake batter. If it seems very runny, let it rest for 5 minutes to thicken.

Using a small cookie scoop (about 1 1/2 tablespoons) or a rounded tablespoon, drop mounds of batter onto the prepared baking sheets, spacing them about 2 inches apart. Try to keep the mounds similar in size so the whoopie pies pair up evenly.

Bake one sheet at a time for 9–11 minutes, or until the tops are set, slightly rounded, and spring back lightly when touched. They should not look wet in the center, but do not bake until dry so they stay soft and cake-like.

Let the cookies cool on the baking sheet for 5 minutes, then transfer them to a wire rack to cool completely. Make sure the cookies are fully cool before filling, or the marshmallow cream will melt and slide out.

In a clean bowl, beat the softened butter for the filling on medium speed for about 2 minutes, until smooth and creamy. Add the sifted powdered sugar, vanilla extract, and a pinch of fine sea salt. Beat on low speed at first, then increase to medium until light and fluffy. Add the marshmallow creme and beat for 1–2 minutes more, until the filling is thick, smooth, and airy. If the filling is too thick, beat in 1–2 tablespoons of heavy cream or milk, a little at a time, until spreadable.

Pair up the cooled cookies by size. Using a piping bag or a small offset spatula, spread about 1–2 tablespoons of marshmallow filling onto the flat side of one cookie. Top with a second cookie, flat side down, and gently press until the filling just reaches the edges. Repeat with the remaining cookies and filling.