In a medium bowl, whisk together the flour, cocoa powder, and salt.

In a large bowl, beat together the butter and confectioners sugar with an electric mixer on medium speed for 3 to 6 minutes. Scrape the sides as needed - the mixture should be light and fluffy.

Reduce the mixer speed to low and slowly add the flour mixture until combined, about 30 seconds.

Using you hands, press the dough in a ball in the bowl. Transfer the dough to a lightly floured surface and knead it until the dough is very smooth, about 3 minutes (the heat from your hands helps the butter to melt a bit and the dough to come together).

Press the dough into a round disk, place another piece of parchment paper on top, and roll it out into a 9-inch circle, about a 1/2 inch thick.

Transfer the dough and the parchment paper to a parchment-lined baking sheet (the double layer of parchment helps to absorb some of the extra grease in the cookies).

Crimp the edges of the circle with your finger and thumb. Use the tines of a fork to poke holes in a circular pattern all over the dough. Then use a small knife to score the dough into 16 wedges (don't cut all the way through the dough).

Cover the chocolate shortbread with plastic wrap and refrigerate for 20 minutes (this helps the cookie to not shrink while baking).

Adjust the oven rack to the middle position and preheat the oven to 300 degrees. Sprinkle the shortbread evenly with the granulated sugar. Bake the cookie for 40 minutes.

When the cookie is done baking, place the baking sheet on a wire rack and use a sharp knife to cut through the scores, separating the wedges, while the cookie is still hot.

Let cool on the baking sheet for 10 minutes and then transfer the shortbread to a wire rack and let it cool before serving, about one hour.