Whisk together the flour, cocoa, and soda in a small bowl; set aside.
Beat the butter on medium until soft and creamy. Add sugars, salt, and vanilla. Beat for 2 minutes.
Add the dry ingredients, pulse on low speed to incorporate flour mixture. Mix until the flour disappears into the dough (the dough will look crumbly).
Turn the dough out onto a work surface and divide it in two. Shape into 2, 9-inch logs. Wrap logs in plastic wrap and refrigerate for at least 3 hours. Don't skip this step. It must be nice and chilled!
Center a rack in the oven and preheat the oven to 325 degrees F. Line two baking sheets with parchment or silicone mats.
Slice the logs into rounds that are 3/4" thick, make sure rounds are "together" (as in, not crumbling), and arrange on the baking sheets.
Sprinkle salt on top of each cookie. Bake one sheet at a time for 11-12 minutes. *The cookies will look somewhat "raw" still, but they're done!
