1. Prepare the Cookie Batter:

• Preheat the oven to 350°F (175°C). Line a baking sheet with parchment paper.
• In a bowl, whisk together flour, cocoa powder, baking soda, and salt.
• In a separate bowl, cream butter, granulated sugar, and brown sugar until light and fluffy.
• Beat in the egg and vanilla extract.
• Alternate adding the dry ingredients and buttermilk to the wet mixture, starting and ending with dry ingredients. Mix until smooth.
• Scoop tablespoon-sized portions of batter onto the prepared baking sheet, spacing them apart.
• Bake for 8–10 minutes, or until the cookies spring back when lightly touched. Let cool completely.

2. Make the Filling:

• Beat softened butter until creamy.
• Add marshmallow fluff, powdered sugar, and vanilla extract.
• Mix until smooth and fluffy. (For cream cheese filling, beat softened cream cheese with butter before adding powdered sugar and vanilla.)

3. Assemble the Whoopie Pies:

• Spread or pipe a generous dollop of filling onto the flat side of one cookie.
• Top with another cookie, pressing gently to seal.
• Repeat until all cookies are paired.
• Chill in the fridge for 15–20 minutes to set the filling.

4. Serve and Store:

• Serve at room temperature for the best texture.
• Store in an airtight container in the fridge for up to 3 days, or freeze for longer storage.