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1
Sift flour, cocoa & baking soda together.
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2
Working with a stand mixer, preferably fitted with a paddle attachment, or with a hand mixer in a large bowl, beat butter on medium speed until soft & creamy. Add both sugars, salt & vanilla extract & beat for 2 minutes more.
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3
Turn off mixer. Pour in dry ingredients; drape a kitchen towel over the stand mixer to stop flying flour & pulse the mixer at low speed about 5 times, a second or two each time. Take a peek; if there's 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 dough as little as possible once flour is added, & don't be concerned if the dough looks a little crumbly. Toss in chocolate pieces & mix only to incorporate.
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4
Turn dough out onto a work surface, gather it together & 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 & refrigerate them for at least 3 hours. (The dough can be refrigerated for up to 3 days or frozen for up to 2 months. If you've frozen the dough, you needn't defrost it before baking - just slice the logs into cookies & bake the cookies 1 minute longer.)
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5
Center a rack in the oven & preheat to 325 u00b0F. Line 2 baking sheets with parchment or silicone mats.
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6
Using a sharp thin knife, slice the logs into rounds 1/2 an 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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7
Bake the cookies 1 sheet at a time for 12 minutes. They won't look done, nor will they be firm, but that's the way they should be. Transfer the baking sheet to a cooling rack & let the cookies rest until they are only just warm, at which point you can serve them or let them reach room temperature.
