In a large mixing bowl, beat the butter and sugar together on medium speed, until smooth (about 1 minute).
Stir in the vanilla and salt.
Scrape the bottom and sides of the bowl with a silicone spatula, and mix in the egg.
Add the dark cocoa powder, and mix on low speed to combine.
Add the flour, a little at a time, until the dough forms a ball and pulls cleanly away from the sides of the bowl (you might not need all the flour*).
Roll out the dough between two sheets of parchment paper, to a thickness of 1/4-inch (rolling pin rings will help you to get an even thickness).
Use a 2-inch diameter fluted round cutter to cut shapes, place on the prepared baking sheets, and bake for 9 to 12 minutes or until crisp around the edges and set in the centers.
Cool completely, and sandwich with creme filling.
To make the creme filling:
In a large mixing bowl, beat the powdered sugar, cream cheese, vanilla, and salt together until smooth.
Recipe Video
Recipe Notes
*If the dough seems sticky, add more flour. If it's too crumbly, it can be moistened back up with a few tablespoons of cream.