Sift the flour, cocoa and baking soda together.
Beat the butter and both sugars together on medium speed until soft, creamy and homogenous, about 3 minutes.
Beat in the salt and vanilla.
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.
Toss in the chocolate pieces and mix to incorporate.
Turn the dough out onto a work surface and gather it together, kneading it if necessary.
Divide the dough in half and shape into logs that are 1 ½ inches in diameter.
Wrap the logs in plastic wrap and freeze for at least 2 hours or refrigerate for at least 3 hours.
Preheat the oven to 325 degrees F and line two baking sheets with parchment paper.
Slice the dough into ½-inch-thick rounds and arrange on the baking sheets.
Bake the cookies for 12 minutes without opening the oven.
Let the cookies rest until they are only just warm or reach room temperature.
