Preheat the oven to 350°F. 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 mixing bowl, cream together the softened butter, granulated sugar, and brown sugar until light and fluffy, about 3-4 minutes.

Add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract.

Gradually add the dry ingredients to the wet mixture, alternating with the milk. Begin and end with the dry ingredients, mixing until just combined.

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

Bake for 10-12 minutes, or until the tops are set and a toothpick inserted comes out clean. Allow to cool on the baking sheets for 5 minutes before transferring to wire racks to cool completely.

While the cookies are cooling, prepare the filling. In a large bowl, beat the softened butter until creamy. Gradually add the powdered sugar, mixing until smooth. Stir in the vanilla extract and heavy cream until the filling is light and fluffy.

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

Serve immediately or refrigerate for up to 3 days.