In a mixing bowl, combine all-purpose flour, dark cocoa powder, baking soda, and salt, then set it aside. In a separate bowl, cream together butter and sugar until light and fluffy.

Gradually mix the dry ingredients into the wet ingredients using a spatula until just combined. Wrap the dough in plastic wrap and chill in the refrigerator for at least 1 hour.

Preheat the oven to 350°F (175°C) and line baking sheets with parchment paper. Scoop dough and roll into balls, flatten them slightly.

Bake the cookies for 10-12 minutes until edges are firm. Let cool on sheets for a few minutes before transferring to a wire rack.

Prepare the filling by blending powdered sugar with butter or coconut cream until smooth. Spread filling on one cookie and sandwich with another.