Preheat your oven to 350°F (175°C). 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 the softened butter and granulated sugar until light and fluffy, about 3-5 minutes.

Beat in the egg and vanilla extract until well combined.

Gradually add the dry mixture to the wet ingredients, mixing until just combined. The dough should be thick and slightly crumbly.

Scoop tablespoon-sized amounts of dough and roll them into balls. Place them on the prepared baking sheets, spacing them about 2 inches apart. Flatten each ball slightly.

Bake for 8-10 minutes, until the edges are set. Let them cool on the baking sheet for 5 minutes before transferring them to a wire rack.

In a mixing bowl, beat the softened butter, powdered sugar, vanilla extract, and heavy cream until creamy and fluffy. Adjust the consistency by adding more cream if necessary.

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