How to Make It
In the bowl of a standing electric mixer fitted with a paddle, beat the butter until creamy. Add the sugar and beat until fluffy. Add the vanilla, then beat in the cocoa on low speed. Beat in the flour and salt; the dough will be very soft.
Divide the dough in half and place each half between 2 large sheets of parchment paper or plastic wrap. Roll out the dough 1/4 inch thick and transfer to 2 baking sheets. Refrigerate until firm, at least 30 minutes.
Preheat the oven to 350°. Working with one piece of dough at a time, remove the top sheet of parchment and invert the dough onto a lightly floured work surface; remove the second sheet of parchment. Using a floured 3-inch cookie cutter, stamp out cookies as close together as possible. Transfer the cookies to parchment paper–lined baking sheets and bake for about 14 minutes, or until firm. Let cool on the baking sheets for 10 minutes, then, using a metal spatula, transfer the cookies to a rack to cool completely. Reroll the scraps, and stamp out more cookies, chilling the scraps between batches.