Step 1: Preheat & Prepare Baking Sheets

1. Preheat oven to 350°F (180°C).
2. Line baking sheets with parchment paper or lightly grease them.

Step 2: Make the Cookie Batter

1. In a large mixing bowl, whisk together flour, cocoa powder, baking soda, baking powder, and salt.
2. In another bowl, cream brown sugar and butter until light and fluffy.
3. Add the egg and vanilla extract, mixing until smooth.
4. Gradually mix in the dry ingredients and buttermilk, alternating between the two.

Step 3: Shape & Bake the Cookies

1. Use a cookie scoop or tablespoon to drop batter onto the baking sheets, spacing them 1-2 inches apart.
2. Bake for 8-10 minutes, rotating trays halfway through.
3. Cool cookies completely on a wire rack.

Step 4: Make the Marshmallow Filling

1. Beat marshmallow fluff, confectioners’ sugar, butter, and vanilla extract until light and fluffy.

Step 5: Assemble the Whoopie Pies

1. Pair cookies of similar size.
2. Pipe or spread a generous layer of filling onto one cookie.
3. Gently press another cookie on top to form a sandwich.
4. Refrigerate for at least 30 minutes before serving for the best texture.