1 1/4 cups all-purpose flour
1/3 cup unsweetened cocoa powder
1/2 teaspoon baking soda
11 tablespoons unsalted butter, at room temperature
2/3 cup (packed) light brown sugar
1/4 cup sugar
1/2 teaspoon fleur de sel or 1/4 teaspoon fine sea salt
1 teaspoon pure vanilla extract
5 ounces bittersweet chocolate, chopped into chips, 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.
Turn off the mixer. Pour in the dry ingredients and pulse the mixer at low speed about 5 times, a second or two each time. If there is still a lot of flour on the surface of the dough, pulse a couple of times morel. 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. 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. (The dough can be refrigerated for up to 3 days or frozen for up to 2 months. If you've frozen the dough, you don't need to defrost it before baking — just slice the logs into cookies and bake the cookies 1 minute longer.)
Getting Ready to Bake:
Center a rack in the oven and preheat the oven to 325 degrees. Line two baking sheets with parchment or silicone mats or aluminum foil
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 — 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. 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.
