1 1/4 c. flour
1/3 c. cocoa powder
1/2 tsp. baking soda
1 stick plus 3 tbsp. butter, at room temperature
2/3 c. packed brown sugar
1/4 c. sugar
1 tsp. vanilla
1/2 tsp. sea salt
5 oz. bittersweet chocolate, chopped into small bits
1. Sift the flour, cocoa and baking soda together in small bowl; set aside. Beat the butter until smooth in bowl of an electric mixer at medium speed. Add the sugars, vanilla and salt; beat 2 minutes. Reduce speed to low; add the flour mixture, mixing until incorporated but still crumbly, and being careful not to overwork the dough. Stir in the chocolate pieces.
2. Turn the dough out onto a smooth work surface; squeeze it so that it sticks together in large clumps. (If you need to, it’s okay to lightly flour the work surface.) Gather the dough into a ball; divide in half. Shape each ball into a log 1 1/2 inches in diameter. Wrap logs in plastic wrap; chill at least 1 hour.
3. Heat oven to 325 degrees. Line two baking sheets with parchment. Gently slice logs into 1/2-inch rounds using a serrated knife (some will crumble; simply press broken bits back onto cookie). Place 1 inch apart on the baking sheets.
4. Bake, one sheet at a time, 14 minutes; cookies will not look done or be firm. Cool on pan 5 minutes; transfer to cooling rack. Cool to room temperature.