||Sift together the flour, cocoa, and baking soda. Set aside.
Using a stand mixer or hand mixer, beat butter on medium speed until soft and creamy. Add brown sugar, granulated sugar, salt, and vanilla extract. Beat for 2 minutes.
Turn off mixer and add dry ingredients. Pulse mixer at low speed about 5 times, a second or two each time. Mix just until flour disappears into the dough. The dough should look a little crumbly. Add chocolate pieces and stir just until mixed.
Turn dough out onto a work surface. Divide dough in half. Shape each half into logs 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°. Line two baking sheets with silicone mats or parchment paper.
Slice 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 sheet at a time) for 12 minutes. These cookies won’t look done or be firm. Cool cookies on pan on a cooling rack. Serve warm or cool completely and store.