Sift together the flour, cocoa, and baking soda. Set aside.
Using a stand mixer or hand mixer, beat the butter on medium speed until soft and creamy. Add the brown sugar, granulated sugar, salt, and vanilla extract. Beat for 2 minutes.
Turn off the mixer and add the dry ingredients. Pulse the mixer at low speed about 5 times, a second or two each time. Mix just until the flour disappears into the dough. The dough should look a little crumbly. Add the chocolate pieces and stir just until mixed.
Turn the dough out onto a work surface. Divide the dough in half. Shape each half into a log about 1 & 1/2 inches in diameter. Wrap each in plastic wrap and refrigerate for at least 3 hours. (The dough may be refrigerated for up to 3 days or frozen for up to 2 months.)
Preheat oven to 325°F (160°C). Line two baking sheets with silicone mats or parchment paper.
Slice the logs into rounds about 1/2-inch thick. These are likely to crack and crumble as they are cut. Squeeze the dough back together and place each cookie on baking sheets, leaving about 1 inch between them.
Bake, one pan at a time, for 12 minutes. These cookies won’t look done or be firm. Cool the cookies on the pan on a cooling rack. Serve warm or cool completely and store.