With a stand mixer fitted with a paddle attachment or with a hand mixer in a large bowl, beat the butter with the brown and granulated sugars, kosher salt, and vanilla until soft and creamy. If you started with cold butter in a stand mixer, you'll need to beat it for several minutes until it's smooth, scraping down the bowl two or three times so that it evenly mixes.

Sift the flour, cocoa and baking soda right into the butter mixture. On low speed, carefully mix the flour mixture into the butter mixture just until the flour disappears into the dough, scraping it down as needed. The dough will look a little crumbly. Add the chocolate pieces and mix only to incorporate.

Turn the dough out onto a large piece of parchment paper and shape it into a log about 2 inches in diameter and 15-16 inches long. Roll the log in the parchment paper and chill for 3 hours, until firm. If you're going to chill it longer such as overnight or for a couple days, wrap the log in plastic wrap too, so it doesn't dry out.

Heat your oven to 325°F (160°C). Line two large baking sheets with parchment paper. Working with a sharp thin knife, slice the logs into rounds that are 1/2-inch thick. The rounds are likely to crack as you're cutting them -- don't be concerned, just squeeze the bits back onto each cookie. Arrange the rounds on the baking sheets, leaving about one inch between them. Sprinkle each cookie with a flake or two of sea salt.

Bake cookies, one sheet a time for even baking, for about 12 minutes. They will be very soft and look completely underbaked, but that's just the way 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 transfer them to a cooling rack.

Cookies keep in an airtight container at room temperature for 1 week, or so I've heard. You can also freeze them tightly packed in a container for up a month. You can freeze the log of unbaked dough, wrapped in plastic, for up to 2 months. No need to defrost before baking. They usually just need 1 more minute of baking time.