How to Make It
In a medium bowl, sift the flour with the cocoa and baking soda. In a large bowl, cream the butter. Add the brown sugar, granulated sugar, fleur de sel and vanilla and beat until combined. Beat in the sifted dry ingredients just until blended; the dough will be fairly crumbly but will hold together. Knead in the chocolate until evenly distributed. Divide the dough in half and transfer to 2 large sheets of plastic wrap. Shape each piece of dough into a 1 1/2-inch-wide log and wrap in the plastic. Refrigerate until firm, at least 2 hours.
Preheat the oven to 350°. Line 4 cookie sheets with parchment paper. Using a sharp, thin knife, cut the logs into 3/8-inch slices and arrange about 1 inch apart on the cookie sheets. If the slices crumble, re-form the cookies, pressing the dough together. Bake the cookies on the middle and lower racks of the oven for about 17 minutes, or until puffed and cracked on top; shift the pans from top to bottom and front to back halfway through. Let the cookies cool on the sheets for 5 minutes, then transfer them to wire racks to cool completely.