Sift the flour, cocoa, and baking soda together and keep the bowl close. Put the butter in the bowl of a mixer fitted with the paddle attachment and beat on medium speed until the butter is soft and creamy. Add both sugars, the salt, and vanilla extract and beat for another minute or two. Reduce the mixer speed to low and add the sifted dry ingredients. Mix only until the dry ingredients are incorporated — the dough will look crumbly, and that’s just right. For the best texture, work the dough as little as possible once the flour is added. Toss in the chocolate pieces and mix only to incorporate.
Turn the dough out onto a smooth work surface and squeeze it so that it sticks together in large clumps, don't be afraid to smoosh it together. Gather the dough into a ball, divide it in half, and working with one half at a time, shape the dough into logs that are 1 1/2 inches (4 cm) in diameter. (Cookie-dough logs can end up with hollow centers, so as you’re shaping each log, flatten it once or twice and roll it up from one long side to the other to get the air out)
Wrap the logs in plastic wrap and chill them for at least 1 hour. (Wrapped airtight, the logs can be refrigerated for up to 3 days or frozen for 1 month.)
Center a rack in the oven and preheat the oven to 325°F (165°C). Line two baking sheets with parchment paper.
Working with a sharp thin-bladed knife, slice the logs into rounds that are 1/2 inch (1.5 cm) thick. (Don’t worry if the rounds break; just squeeze the broken-off bit back onto the cookie.) Place the cookies on the parchment-lined sheets, leaving about 1 inch (2.5 cm) spread space between them.
If you want to up your salt game, sprinkle a tiny bit of Fleur de Sel on each cookie before baking. It adds a tiny wow factor to the look of the cookie too.
Bake one sheet of cookies at a time, and bake each sheet for 12 minutes. The cookies will not look done, nor will they be firm, but that’s just the way they should be. Transfer the baking sheet to a cooling rack and let the cookies stand until they are only just warm or until they reach room temperature.