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1
Sift the flour, cocoa and baking soda together. In a large bowl, beat the butter on medium speed until soft and creamy. Add both sugars, the salt and vanilla extract and beat for 2 minutes more.
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2
Pour in the flour start the mixer at low speed. Beat a second or two - if there is still a lot of flour on the surface of the dough, Beat a couple of times more; 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, Toss in the chocolate pieces and mix only to incorporate.
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3
Turn the dough out onto a work surface, gather it together and divide it in half. Working with one half at a time, shape 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.
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4
Center a rack in the oven and preheat the oven to 325 degrees F. Line two baking sheets with parchment or silicone mats.
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5
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 1 inch between them.
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6
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.
