Preheat your oven to 350°F (175°C) and line baking sheets with parchment paper.
In a large bowl, whisk together the flour, cocoa powder, baking powder, baking soda, and salt.
In another bowl, cream together the softened butter and granulated sugar until light and fluffy, about 3-5 minutes.
Add the eggs one at a time to the butter mixture, mixing well after each addition, then stir in the vanilla extract.
Gradually add the dry ingredients to the wet mixture, mixing just until combined to avoid overworking the batter, which can lead to tough cakes.
Using a small cookie scoop or spoon, drop heaping tablespoons of batter onto the prepared baking sheets, spacing them about 2 inches apart.
Bake in the preheated oven for 10-12 minutes or until a toothpick inserted comes out clean.
Once baked, remove the pies from the oven and let them cool on the baking sheets for a few minutes before transferring to a wire rack to cool completely.
In a separate bowl, combine marshmallow fluff, powdered sugar, and salt, mixing until smooth and airy.
Spoon a dollop of the filling onto the flat side of one cake, then gently press another cake on top to create a sandwich.
Repeat the process until all cakes are filled and assembled, then enjoy your freshly made mini whoopie pies!