Preheat the oven to 375 degrees F, and line baking sheets with parchment paper.
In a large mixing bowl, beat the sugar and butter together on medium speed, until smooth (about 1 minute).
Scrape the bottom and sides of the bowl with a silicone spatula, and mix in the in the egg and vanilla.
Add the dark cocoa powder, cornstarch, and salt, 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 ¼-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 until crisp around the edges and set in the centers (approx. 9 to 12 minutes).
Cool completely, and sandwich with creme filling.
In a large mixing bowl, beat the powdered sugar, cream cheese, vanilla, and salt together until smooth.
