Preheat the oven to 350°F (175°C).

In a mixing bowl, whisk together flour, cocoa powder, baking soda, and salt.

In another bowl, cream together the softened butter and granulated sugar until light and fluffy.

Beat in the egg and vanilla extract until well combined.

Gradually add the dry ingredients to the wet ingredients, mixing until just combined.

Scoop tablespoon-sized amounts of dough and roll into balls.

Place the balls on a baking sheet lined with parchment paper, spacing them about 2 inches apart.

Flatten each ball slightly with the bottom of a glass.

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

Remove from the oven and let cool on the baking sheet for 5 minutes before transferring to a wire rack.

In a mixing bowl, beat the softened butter until creamy.

Gradually add powdered sugar and vanilla extract, mixing until smooth and fluffy.

Once the cookies are completely cool, spread a generous amount of cream filling on the flat side of one cookie.

Top with another cookie, flat side down, to create a sandwich.

Repeat with remaining cookies and filling.