1. Whisk together the flour, cocoa and baking soda in a small bowl; set aside.
2. Beat the butter on medium speed until soft and creamy. Add both sugars, the salt and vanilla extract and beat for 2 minutes more.
3. With the mixer off, add the dry ingredients. Turn the mixer on and off low speed (pulse) for a second or two about 5 times so that the flour mixture gets incorporated. Then mix on low speed for about 30 seconds, just until the flour disappears into the dough (the dough will look crumbly).
4. Turn the dough out onto a work surface and divide it in two. Shape each half into a 9-inch log. Wrap the logs in plastic wrap and refrigerate for at least 3 hours. (The dough can be refrigerated up to 3 days or frozen for up to 2 months. If you've frozen the dough, you don't need to defrost before baking - just slice the logs into cookies and baking the cookies 1 minute longer.)
5. Center a rack in the oven and preheat the oven to 325 degrees F. Line two baking sheets with parchment or silicone mats.
6. Using a sharp thin knife, slice the logs into rounds that are ½-inch thick. Arrange the rounds on the baking sheets, leaving about 1 inch between them. Sprinkle a small amount of extra salt on top of each round.
7. Bake the cookies one sheet at a time for 12 minutes - they won't look done, and won't be firm, but that's how they should be. Transfer the baking sheet to a cooling rack and let the cookies rest until they are only just warm, at which point you can serve them or let them reach room temperature. Store the cookies in an airtight container at room temperature.