Ingredients:
1 1/4 c. all-purpose flour
1/3 c. unsweetened cocoa powder
1/2 tsp. baking soda
1 stick + 3 Tbsp. (11 Tbsp. total) unsalted butter, at room temperature
2/3 c. (packed) light brown sugar
1/4 c. sugar
1/2 tsp. fleur de sel (or 1/4 tsp. fine sea salt)
1 tsp. vanilla extract
5 oz. bittersweet chocolate, chopped into chip (or 3/4 c. store-bought mini chocolate chips)
Sift the flour, cocoa, and baking soda together.
Working with a stand mixer, preferably 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 flour, drape a kitchen towel over the stand mixer to protect yourself and your kitchen from flying flour and pulse the mixer at low speed about 5 times, a second or two each time. Take a peek — if there is 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 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.
Getting ready to bake: Center a rack in the oven and preheat the oven to 325 F. Line two baking sheets with parchment or silicone mats.
Working with 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 one 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.