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

In a mixing bowl, sift together the flour, cocoa powder, baking soda, baking powder, and salt. Set this mixture aside.

In a large mixing bowl, cream the softened butter and granulated sugar together until light and fluffy.

Add the egg and vanilla extract into the butter mixture. Mix well until fully combined.

Gradually incorporate the dry mixture into the wet ingredients. Stir until you form a smooth dough.

Using a tablespoon or a cookie scoop, drop balls of dough onto the prepared baking sheets, leaving about 2 inches between each one.

Bake for 10-12 minutes, or until the cookies look set and the edges are slightly firm. Remember that they will continue to firm up as they cool.

Remove the cookies from the oven and let them cool on the baking sheets for 5 minutes before transferring them to wire racks to cool completely.

While the cookies are cooling, prepare the filling. In a bowl, beat together the softened butter and powdered sugar until creamy.

Add the vanilla extract and mix well. If the filling seems too thick, add milk one tablespoon at a time until you reach the desired consistency.

Once the cookies have cooled, spread a generous layer of filling on the flat side of one cookie and sandwich it with another cookie.