Preheat the oven to 350degrees F. Line 2 baking sheets with parchment paper

Sift together the flour, cocoa powder, and baking soda into a bowl and set aside.

In the bowl of a stand mixer fitted with the paddle attachment, beat together the butter, sugar, salt, and vanilla until lightened, about 4 minutes.

Add the dry ingredients and grated chocolate and mix just to combine.

If the dough is soft, wrap it tightly in plastic wrap and refrigerate for 30 minutes. (The dough will keep, wrapped in plastic, for up to 1 week in the refrigerator or 3 months in the freezer.) Otherwise, roll out the dough ½ inch thick on a lightly floured work surface into a 6-by-6-inch square.

Using a ruler, square the edges as much as possible. Using a sharp knife or pizza cutter, cut the dough into 1-inch squares. Place them 2 inches apart on the prepared baking sheet. Sprinkle lightly with flaky sea salt

Bake the cookies until they are firm, 10 to 12 minutes. Remove to a wire rack to cool. Store in airtight containers for up to 2 weeks.

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