Grate the chilled chocolate with a fine grater or rasp and set aside.

Whisk together the flour, cocoa powder, baking soda and sea salt.

Beat the butter and sugar in a medium bowl with an electric mixer until just combined. Mix in the yolk. Add the dry ingredients to the butter and beat lightly together until just combined but still crumbly. Fold in grated chocolate with a spatula. Bring the dough together by lightly squeezing in your hands; but don't knead or overwork, as the secret to these cookies is their delicate, sandy texture.

Divide the dough in half. Lay half the dough on a long sheet of waxed paper and shape into a log along the width of the waxed paper, leaving some space at each end. Pull the paper over the top of the log. Grip the edge of the top piece of paper, and use a straight, firm edge, like a ruler or the edge of a pan, to press gently against the edge of the dough where the papers come together to create a solid, firm round log. Repeat with remaining dough and refrigerate for at least 1 hour. (To keep logs round store inside an empty paper towel roll.)

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

Slice logs into 1/2-inch thick rounds with a sharp, thin knife. Divide rounds onto the prepared sheets, leaving about 1 inch between them, and refrigerate for 15 minutes.

Bake until cookies smell fragrant with a full cocoa aroma and set on the outside, about 12 to 14 minutes. Remove from the oven and let cool on the pans, about 5 minutes.

Transfer cookies to a rack to cool completely. Serve.

Busy baker's tips: Dough can be made and frozen for up to 2 weeks. Store baked cookies in an airtight container for up to 5 days. Baked cookies can be wrapped in plastic wrap, then aluminum foil and frozen for up to 2 weeks.

Cook's note: For super uniform cookies, place each sliced disk of dough in a muffin tin and bake. The cookies will be chewier, less sandy this way.

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