Preheat your oven to 350°F (175°C) and line two baking sheets with parchment paper.

In a medium bowl, whisk together the flour, cocoa powder, baking soda, baking powder, and salt. Set aside.

In a large bowl, cream together the softened butter, granulated sugar, and brown sugar until light and fluffy, about 2-3 minutes.

Beat in the egg and vanilla extract until well combined.

Gradually add the dry ingredients to the wet ingredients, mixing until just combined.

Using a tablespoon or cookie scoop, drop rounded balls of dough onto the prepared baking sheets, spacing them about 2 inches apart.

Bake for 10-12 minutes, or until the edges are set. Allow to cool on the baking sheets for 5 minutes before transferring to a wire rack to cool completely.

While the cookies are cooling, prepare the filling by beating together the powdered sugar, softened butter, vanilla extract, and enough milk to achieve a smooth, spreadable consistency.

Once the cookies are completely cool, spread a generous amount of filling on the flat side of one cookie and sandwich it with another cookie, flat side down. Repeat with the remaining cookies.