1¼ cups all-purpose flour
⅓ cups unsweetened cocoa powder
½ teaspoon baking soda
1 stick plus 3 tablespoons unsalted butter, at room temperature
⅔ cup (packed) light brown sugar
¼ cup sugar
½ teaspoon fleur de sel or ¼ teaspoon fine sea salt
1 teaspoon pure vanilla extract
5 ounces bittersweet chocolate, chopped into chips or a generous ¾ cup store-bought mini chocolate chips
Sift the flour, cocoa and baking soda together.
Working with a stand mixer, fitted with a paddle attachment, beat the butter on medium speed until soft and creamy. Add both sugars, salt and vanilla extract and beat for 2 minutes more.
Turn off the mixer. Pour in the dry ingredients. On low speed, mix for about a minute, 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 divid it in half. Working with one half at a time, shape the dough into logs that are 1½ 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 needn't defrost it before baking— just slice the logs into cookies and bake the cookies 1 minute longer.)
Center a rack in the oven and preheat the oven to 325ºF. Line two baking sheets with parchment or silicone mats.
Using a sharp thin knife, slice the logs into rounds that are ½-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 — 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.