In a medium bowl, sift together flour, cocoa, and baking soda. Set aside.
In a mixer bowl fitted with the paddle attachment, beat the butter on medium speed for 30 seconds until creamy. Add sugars, salt, and vanilla and beat for 2 more minutes until smooth. Add flour mixture and grated chocolate, and beat on low speed just until combined and the mixture is crumbly with large pieces of dough. Work the dough as little as possible, being careful not to over mix.
On a lightly floured surface, work the dough until it comes together, then divide in half. Shape each half of dough into a log that is 1-1.5 inches in diameter. Wrap logs in plastic wrap and refrigerate or freeze (my preference) for at least 30 minutes. Dough can be left in the refrigerator for up to 3 days or in the freezer for 2 months.
Preheat oven to 350F/180C. Line 2 pans with parchment paper.
Once the dough is cold enough, use a very sharp knife to slice the log into 1/2-inch thick rounds. If the dough is too crumbly and breaks, leave it for a few minutes to soften or just squeeze any broken bits back into each cookie.
Arrange cookies on prepared baking sheets, placing them 1-2 inches apart from each other. Sprinkle lightly with granulated sugar. Bake for 10-12 minutes. When done, the cookies should still appear quite soft, not firm. Allow cookies to cool on a wire rack.
Serve cookies warm or at room temperature.
Store cookies at room temperature in an airtight container for up to 5 days. Cookies can be frozen for up to 2 months.