Preheat your oven to 350°F (175°C) and prepare two baking sheets by lining them with parchment paper.

In a large mixing bowl, combine all-purpose flour, unsweetened cocoa powder, baking powder, and salt. Whisk them together thoroughly.

In a separate bowl, cream the granulated sugar and softened unsalted butter together using an electric mixer on medium speed until light and fluffy, about 2–3 minutes.

Beat in the eggs one at a time, followed by the vanilla extract and milk. Mix until a smooth batter forms.

Gradually add the dry ingredients to the wet mixture, stirring gently until just combined. Avoid overmixing.

Using a cookie scoop or spoon, drop rounded tablespoons of batter onto the prepared baking sheets.

Bake for 10–12 minutes, or until the cake springs back lightly when pressed.

Carefully remove the Whoopie Pies from the oven and let them cool on the baking sheets for about 5 minutes.

Whip together the heavy cream and marshmallow fluff in a mixing bowl until smooth and fluffy.

Once the cakes are completely cool, spread or pipe the creamy filling onto the flat side of each cake and sandwich with another cake.

Serve immediately or store in an airtight container at room temperature for up to 5 days.