Sift the flour, cocoa and baking soda together.

Working with a stand mixer fitted with a paddle attachment, beat the butter on medium speed until soft and creamy. Add both sugars, salt, and vanilla and beat for 2 minutes more.

Turn off the mixer. Pour in the dry ingredients, drape a kitchen towel over the stand mixer to protect yourself and your kitchen from flying flour and pulse the mixer at low speed about 5 times, a second or two each time. Take a peek — if there is still a lot of flour on the surface of the dough, pulse a couple of times more; if not, remove the towel. Continuing at low speed, mix for about 30 seconds more, just until the flour disappears into the dough — for the best texture, work the dough as little as possible once the flour is added, and don't be concerned if the dough looks a little crumbly. Toss in the chocolate pieces and mix only to incorporate.

Turn the dough out onto a work surface, gather it together and divide it in half. Shape each half of the dough into logs that are 1 1/2 inches in diameter. Wrap the logs in plastic wrap and refrigerate them for at least 3 hours or up to 3 days. The dough can also be frozen for up to 2 months. If you freeze it, just add 1 minute to the baking time.

When you are Ready to Bake:

Center a rack in the oven and preheat the oven to 325. Line two baking sheets with parchment or silicone mats.

Using 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 1 inch between them.

Bake the cookies one sheet at a time for 12 minutes — they won't 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 rest until they are only just warm, at which point you can serve them or let them reach room temperature.