Preheat Oven: Preheat oven to 350°F (175°C). Line two baking sheets with parchment paper.
Mix Dry Ingredients: In a medium bowl, whisk together flour, cocoa powder, baking soda, and salt.
Cream Butter and Sugars: In a large bowl, beat butter, granulated sugar, and brown sugar until light and fluffy. Add egg and vanilla, mixing well.
Combine Batter: Alternately add dry ingredients and buttermilk to the butter mixture, beginning and ending with dry ingredients. Mix until just combined.
Pipe or Scoop: Using a cookie scoop or spoon, drop about 2 tablespoons of batter per cookie onto prepared baking sheets, spacing 2 inches apart.
Bake: Bake for 10–12 minutes, or until a toothpick inserted comes out clean. Cool on baking sheets for 5 minutes, then transfer to a wire rack to cool completely.
Prepare Filling: In a medium bowl, beat butter until creamy, then add marshmallow fluff and vanilla. Mix until smooth.
Assemble Whoopie Pies: Spread or pipe filling onto the flat side of one chocolate round, sprinkle with graham cracker crumbs, and top with another chocolate round to form a sandwich.
Serve: Enjoy immediately or store for later.