Sift the flour, cocoa and baking soda together. Set aside.

In the bowl of a standing mixer, beat the butter on medium speed until smooth and pale yellow. Add the sugars, salt and vanilla and mix another 2 minutes.

Turn off the mixer. Pour in the sifted dry ingredients, drape a kitchen towel over the mixer to protect anything around it from flying flour. Mix at low speed about 5 times for a second or two each time. Then mix about 30 seconds more. Mix just until the flour disappears into the dough. Work the dough as little as possible. It may look a little crumbly… no worries. Toss in the chocolate pieces and mix just until incorporated.

Turn the dough out onto a work surface and gather it together and divide it in half. Shape into logs that are 1 1/2 inches in diameter. Wrap the logs in plastic wrap and refrigerate for at least 3 hours. (The dough can be refrigerated for up to 3 days or frozen for up to 2 months.

Preheat the oven to 325°F. Line two baking sheets with parchment paper or silicone mats.

Using a sharp knife, slice the logs into rounds that are about 1/2 inch thick. (They may crack a bit as you’re cutting them. No worries. Just squeeze the chocolatey bits back onto the cookie). Arrange the rounds on the baking sheets, leaving about 1 inch between them.

Bake each batch, one sheet at a time, for 12 minutes. They won’t look done, they won’t be firm at this point. They’ll firm up and become sandy/crumbly as they cool. Transfer the baking sheet to a cooling rack.