In a large bowl, cream together 1 cup (225 grams) of softened butter with the white sugar and salt, until light and fluffy.
Beat in eggs until fully incorporated.
Sieve together the flour, cocoa powder, and baking soda into the mix. Blend well.
Add the dry ingredients to the wet ingredients, and mix together until combined.
Turn the dough out onto your surface and push together into a flat square. Wrap the dough in plastic wrap and refrigerate for 1 hour.
Preheat oven to 325˚F (160˚C).
Remove the dough from the fridge, and for ease of rolling out, divide the dough into 4 (this way the dough stays cool longer and prevents the need of re-chilling later on).
To roll out the dough, place a quarter of the dough between two sheets of parchment paper (alternatively the dough can be rolled out on a lightly floured surface - though the parchment is preferred as it prevents any additional flour from altering the color/appearance of the dark cookies). Roll the dough between the two sheets of parchment to ¼-inch (½ cm) thickness.
Using a small round cookie cutter (alternatively, we found the rim of a champagne glass to be a perfect size!) cut the dough into individual rounds and place on a large parchment-lined baking sheet, leaving at least ½-inch (1¼ cm) between each cookie.
Pack together and re-roll out any scraps to cut additional cookies. Repeat with this process with each remaining ¼ of the dough.
Bake in a preheated oven for 15 minutes. If baking multiple trays at the same time, be sure to rotate the pans in the oven halfway through. Remove and transfer cookies to a cooling rack to cool completely.
To make the filling, combine ½ cup (115 grams) butter, powdered sugar, and vanilla in a medium mixing bowl. Beat together until light and fluffy.
Assemble the cookies by spreading a generous scoop of the icing onto one of the cookies and sandwiching it with another. Give it a light squeeze and scrape any excess off to clear and even out the sides.