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1
In a large bowl, whisk together the flour, cocoa powder and baking soda.
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2
With a stand mixer fitted with the paddle attachment, or with a hand mixer in a large bowl, beat the butter on medium speed until soft and creamy. Add both sugars, salt and vanilla extract and beat for 2 minutes more until fully incorporated. Pour in the dry ingredients and mix, on low speed, until the flour just disappears into the dough. 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 (it's ok if it looks a bit crumbly) and divide it in half. Working with one half at a time, shape the dough into logs that are about 1 1/2 inches in diameter. Wrap the logs in plastic wrap and refrigerate them for at least 3 hours (see note).
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4
When ready to bake, preheat the oven to 325u00b0F. Line two baking sheets with parchment paper. Set aside.
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5
Using a sharp knife, slice the logs into rounds that are 1/2 inch thick. Arrange the rounds on the prepared baking sheets, leaving about 1 inch between them. Bake the cookies, one sheet at a time, for about 12 minutes. Transfer the baking sheet to a cooling rack, sprinkle with some Fleur de sel (or another coarse sea salt) and let the cookies rest for at least 10 minutes before serving them.
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6
Note: 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 and bake the cookies 1 minute longer.
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7
Recipe adapted from Dorie Greenspan via Food52.
