Preheat your oven to 350°F (175°C). Line a baking sheet with parchment paper for easy cookie removal.

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

In a large mixing bowl, combine the softened butter and granulated sugar. Using an electric mixer, cream these ingredients together until the mixture is fluffy and light in color.

Beat in the egg and vanilla extract until fully incorporated.

Gradually add the dry ingredient mixture to the wet ingredients. Mix until just combined—try not to overwork the dough!

With a tablespoon or a cookie scoop, portion out the dough and place them on the prepared baking sheet. Flatten each dough ball slightly since they won't spread much during baking.

Bake in the preheated oven for about 10 to 12 minutes, or until the edges are firm.

Remove from the oven and allow them to cool completely on a wire rack.

While the cookies cool, prepare the cream filling by mixing the powdered sugar, cocoa powder, softened butter, and vanilla extract in a medium bowl until smooth.

Once the cookies are cool, spread a generous amount of the cream filling onto the flat side of one cookie and sandwich it with another cookie.