For the cookies: Preheat the oven to 325 degrees F. Line two baking sheets with parchment paper.
In a small bowl, whisk together the flour, cocoa powder, salt and baking soda. In a stand mixer fitted with a paddle attachment, cream together the butter and granulated sugar until light and fluffy. Mix in the vanilla extract. With the mixer running on low speed, add the flour mixture and beat until just combined; it will still be a bit crumbly. Pour the mixture onto a work surface and give it a few kneads to bring it all together. Divide in half, shape into flat discs, wrap in plastic wrap and place in the refrigerator for an hour (or up to two days).
Dust your surface and the top of the dough with plenty of cocoa powder. Roll out the dough to 1/2 inch thickness and cut out 2 1/2-inch rounds. Transfer them to the baking sheets, 1 inch apart (using a small offset spatula helps with this step). Re-roll the scraps and cut out more rounds.
Bake the cookies until the tops are no longer shiny, about 20 minutes. Cool on the pan for 5 minutes, then remove to a wire rack to cool completely.
For the filling: With a stand mixer fitted with a paddle attachment, beat together the butter, powdered sugar and salt until combined. Fill a piping bag with the filling. Place half of the cookies on a plate or work surface. Pipe a blob of filling onto the tops of each of these cookies and then place another cookie on top, pressing slightly. Refrigerate for a few minutes to allow the filling to firm up. Enjoy! Store in an airtight container in the refrigerator for up to a week.