Preheat your oven to 350°F (175°C) and line two baking sheets with parchment paper.

1. In a medium bowl, whisk together the flour, baking soda, salt, and cocoa powder. Set aside.
2. In a large mixing bowl, beat together the granulated sugar, brown sugar, and softened butter until creamy and smooth.
3. Add the egg and vanilla extract, beating until well combined.
4. Gradually add the dry ingredients to the wet mixture, alternating with the buttermilk. Mix until just combined (don’t overmix).
5. Use a spoon or cookie scoop to drop rounded tablespoons of dough onto the prepared baking sheets, spacing them about 2 inches apart.
6. Bake for 10-12 minutes or until a toothpick inserted into the center of a cookie comes out clean. Allow the cookies to cool on the baking sheets for a few minutes, then transfer them to a wire rack to cool completely.
7. While the cookies cool, make the filling by beating together the softened butter, powdered sugar, marshmallow fluff, vanilla extract, and a pinch of salt until smooth and fluffy.
8. Once the cookies are completely cooled, turn half of them over so the flat side is facing up. Spread a generous amount of the marshmallow filling on the flat side of each cookie.
9. Top with another cookie, flat side down, to create a sandwich. Press gently to spread the filling to the edges.
10. Serve immediately or store in an airtight container at room temperature for up to 3 days. Enjoy!