Preheat the oven to 350 degrees F.
Butter a 9 1/2 inch pan with removable bottom, then sprinkle it with flour. Set aside.
Combine butter, sugar, cocoa, and salt in a medium size bowl. Beat with an electric mixer, starting on low at first then gradually increase the speed to medium, mixing until just combined. Scrape the sides of the bowl. Mix briefly.
Add the flour and mix the ingredients on low speed. Mix until the dough clumps, takes about 1 minute or so. Scrape the sides of the bowl. Mix briefly.
Transfer mixture into the pan, press gently into the bottom. No need to go up the sides, just cover the bottom evenly.
Place in the oven on the middle rack, and cook for about 20 to 25 minutes. You want the top of the shortbread to lose it's wet appearance, and the dough to slightly pull away from the sides of the pan a little bit. That lets you know it's done.
Remove from oven. Set aside.