Whisk the flour, cocoa and baking soda together until uniform in color with no lumps.
130 g all-purpose flour, 30 g unsweetened cocoa powder, 1/3 tsp baking soda
Working with a stand mixer fitted with the paddle attachment, or in a large bowl with a hand mixer, beat the butter and both sugars together on medium speed until soft, creamy. Beat in the vanilla, salt and espresso powder.
113 g unsalted butter, 120 g light brown sugar, 38 g sugar, 1/2 tsp sea salt, 1/2 tsp espresso powder, 3/4 tsp pure vanilla extract
Turn off the mixer, add all the dry ingredients and pulse a few times to start the blending. Turn the mixer to low and beat until the dough forms big, moist curds.
Add in the chocolate pieces and mix just to incorporate.
107 g bittersweet chocolate
Turn the dough out onto a work surface and gather it together, kneading it if necessary to bring it together. Divide the dough in half. Shape the dough into logs that are 1 1/4 inches in diameter. Wrap the logs in waxed paper and freeze them for at least 2 hours or refrigerate them for at least 3 hours.
Preheat the oven to 325°F. Line two baking sheets with parchment paper or silicone baking mats.
Working with one log at a time and using a long, sharp knife, slice the dough into 1 cm (about 1/3 inch) thick rounds. (The rounds might crack as you’re cutting them - squeeze the bits back onto each cookie.) Arrange the rounds on the baking sheets, leaving about 2 inches between them.
Bake the cookies for 9 minutes - they won’t look done or be firm, but do not bake them any longer. Let the cookies cool for several minutes on the pan, transfer to wire rack to cool completely.
Notes
Mix or knead the dough only enough to bring it together and allow it to be shaped into solid logs. Overworking the dough will alter the texture, making them less "sandy" or even tough if it is really overworked, but underworking it will make it more difficult to shape.