1 1/4 cups all-purpose flour
1/3 cup Valrhona unsweetened cocoa powder
1/2 teaspoon baking soda
11 tablespoons unsalted butter, at room temperature and cut into chunks
2/3 cup (packed) light brown sugar
1/4 cup sugar
1/2 teaspoon fleur de sel (or 1/4 t fine sea salt)
1 teaspoon pure vanilla extract
5 oz great-quality bittersweet chocolate, chopped into irregular-sized bits, or a generous 3/4 cup store-bought mini chocolate chips
Sift the flour, cocoa, and baking soda together.
Working with a stand mixer fitted with a paddle attachment or with a hand mixer 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. You want the mixture to be smooth, but not airy. (With cookies, it’s better to beat less than more.)
Turn off the mixer and pour in all the dry ingredients. Pulse the mixer a few times to start the blending. When the risk of flying flour has passed, turn the mixer to low and beat only until the flour and cocoa disappear 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 to incorporate. (This is an unpredictable dough. Sometimes it’s crumbly and sometimes it comes together and cleans the sides of the bowl. No matter what, the cookies always come out great.)
Turn the dough out onto a work surface and gather it together—if it’s really crumbly or not easily gatherable, knead it a bit (it can take it). Divide the dough in half.
Working with one half at a time, shape the dough into logs that are 1 1/2 inches in diameter. Don’t worry about the length—get the diameter right and the length will follow. (If you get a hollow in the logs, it happens; just start over.) Wrap the logs in plastic wrap and refrigerate them for at least 3 hours, or freeze them (my preference) for at least 2 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 and bake the cookies 1 minute longer.)
When you’re ready to bake, center a rack in the oven and preheat the oven to 325°F. Line two baking sheets with parchment paper 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—don’t open the oven; just let them bake. When the buzzer rings, 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 munch them or let them reach room temperature. (I think the texture’s more interesting at room temperature.)
Storing: Packed airtight, the cookies will keep at room temperature for up to 3 days; they can be frozen for up to 2 months, as can the logs of dough.