Preheat your oven to 350°F (175°C) and line two baking sheets with parchment paper.
In a medium bowl, whisk together the flour, cocoa powder, baking soda, and salt until well combined.
In a separate small bowl, mix the buttermilk and vanilla extract.
Using a stand mixer or hand mixer, cream together the butter and brown sugar until light and fluffy, about 3 minutes. Beat in the egg until fully incorporated.
Gradually add the dry ingredients to the butter mixture, alternating with the buttermilk mixture, beginning and ending with the dry ingredients. Mix just until combined after each addition.
Using a cookie scoop or tablespoon, drop rounded portions of dough onto the prepared baking sheets, spacing them about 2 inches apart.
Bake for 10-12 minutes until the tops spring back when lightly touched.
Allow to cool on the baking sheets for 5 minutes before transferring to a wire rack to cool completely.
While the cookies cool, prepare the filling. In a large bowl, beat the softened butter until creamy.
Gradually add the confectioners' sugar, beating until smooth.
Add the marshmallow fluff, vanilla, and salt, then beat on medium-high speed until light and fluffy, about 2-3 minutes.
Once cookies are completely cool, match them up in pairs of similar size.
Spread or pipe a generous amount of filling (about 2 tablespoons) onto the flat side of one cookie from each pair.
Gently press the second cookie, flat side down, onto the filling to create a sandwich.
For the authentic Amish touch, allow the assembled whoopie pies to rest for 1 hour before serving.