Preheat your oven to 350°F (175°C).
In a bowl, whisk together the flour, cocoa powder, baking powder, baking soda, and salt.
In another bowl, cream the softened butter and granulated sugar until light and fluffy.
Add the egg and vanilla extract to the butter mixture; mix well.
Gradually add the dry ingredients to the wet mixture; blend until smooth.
Using a cookie scoop, drop rounded tablespoons of batter onto a lined baking sheet.
Bake for 10-12 minutes or until the edges are set.
Allow the cookies to cool completely on a wire rack.
In a separate bowl, mix marshmallow fluff and powdered sugar until smooth.
Spread or pipe the filling onto the flat side of one cookie and sandwich with another.