Preheat the oven to 350°F (175°C). Line 2 large baking sheets with parchment paper or silicone baking mats to prevent sticking and keep the cookie bottoms tender.

In a medium bowl, whisk together the flour, cocoa powder, baking soda, baking powder, and fine sea salt until evenly combined with no visible streaks of flour or cocoa.

In a large mixing bowl using a hand mixer or stand mixer, beat the softened butter, granulated sugar, and light brown sugar on medium speed for 2–3 minutes, until lighter in color and slightly 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 mixture may look slightly curdled; it will smooth out once the dry ingredients and buttermilk are added.

On low speed, add about one-third of the dry ingredient mixture to the butter mixture and mix just until combined. Pour in half of the buttermilk and mix gently. Add another third of the dry ingredients, then the remaining buttermilk. Finish with the last third of the dry ingredients, mixing just until a thick, scoopable batter forms. Do not overmix.

Using a 1 1/2-tablespoon cookie scoop or rounded tablespoon, drop mounds of batter onto the prepared baking sheets, leaving at least 2 inches between each mound to allow for spreading. For more even rounds, lightly smooth the tops with a damp finger or the back of a spoon. If desired, lightly sprinkle the tops with coarse sea salt.

Bake one sheet at a time for 10–12 minutes, or until the tops look set and spring back lightly when touched and a toothpick inserted into the center comes out mostly clean. Let the cookies cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely before filling.

While the cookies cool, prepare the marshmallow cream filling. In a clean bowl, beat the softened butter for the filling on medium speed for 1–2 minutes, until smooth and creamy.

Gradually add the powdered sugar to the butter, mixing on low speed at first to avoid dusting, then increasing to medium. Beat for 2–3 minutes until light and fluffy.

Add the marshmallow creme, vanilla extract, and a pinch of fine salt to the bowl. Beat until the filling is thick, smooth, and billowy. If the filling is too soft, beat in an additional 2–4 tablespoons of powdered sugar. If it is too stiff, add 1–2 teaspoons of milk or cream to loosen slightly.

Once the cookies are completely cool, sort them into pairs of similar size. Flip half of the cookies upside down to be the bottoms. Pipe or spoon about 1 1/2–2 tablespoons of marshmallow filling onto each bottom cookie, then gently press a second cookie on top until the filling just reaches the edges.

If desired, roll the exposed filling edges of each assembled whoopie pie in sprinkles or mini chocolate chips while the filling is still soft so they adhere.

Serve immediately, or place the whoopie pies in a single layer in an airtight container. For the best texture, let them rest for a few hours or overnight so the cookies soften slightly from the filling.