Preheat your oven to 350°F (180°C) and line baking sheets with parchment paper.

In a medium bowl, whisk together the all-purpose flour, cocoa powder, baking soda, and salt. Set this mixture aside.

In a large bowl, beat the softened butter, granulated sugar, and brown sugar until the mixture is light and creamy using a mixer.

Add the egg and vanilla extract to the butter mixture and mix until fully combined.

Gradually add the flour mixture to the wet ingredients, mixing until everything is just combined. The dough should come together but remain slightly sticky.

Using a small cookie scoop or your hands, form small balls of dough, roughly one inch in diameter, and place them on the prepared baking sheets about two inches apart.

Bake in the preheated oven for about 10-12 minutes or until the edges look set and the tops are slightly cracked.

Remove from the oven and allow the cookies to cool on the baking sheets for about 5 minutes, then transfer them to a wire rack to cool completely.

While the cookies cool, prepare the filling by mixing powdered sugar, heavy cream, and vanilla extract in a medium bowl until smooth and creamy.

Once the cookies are completely cooled, spread a heaping tablespoon of the cream filling on the flat side of one cookie and sandwich it with another cookie. Repeat until all cookies are filled.