Make the Chocolate Cookies:
Preheat oven to 350°F (175°C). Line two baking sheets with parchment paper.
In a medium bowl, whisk flour, cocoa powder, baking soda, baking powder, and salt. Set aside.
In a large mixing bowl, beat butter, granulated sugar, and brown sugar until creamy and fluffy (about 2–3 minutes).
Add egg and vanilla extract, beating until combined.
Alternate adding the flour mixture and buttermilk to the butter mixture, beginning and ending with the flour. Mix until just combined—don’t overmix.
Using a cookie scoop, drop 2-tablespoon mounds of batter onto the prepared baking sheets, spacing 2 inches apart.
Bake for 10–12 minutes, or until the tops spring back when gently pressed.
Let cookies cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely.
Make the Cream Filling:
In a large bowl, beat butter until smooth.
Add powdered sugar and mix until fluffy.
Beat in marshmallow creme and vanilla extract until light and creamy.
Assemble the Whoopie Pies:
Spread or pipe filling onto the flat side of half the cookies.
Top with remaining cookies, pressing gently to sandwich together.