Method:

1) Grate the chilled chocolate with a fine grater 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 and then mix in the egg yolk.

2) Add the dry ingredients to the butter mixture and beat lightly together until crumbly. Fold in the grated chocolate with a spatula and bring the dough together by lightly squeezing in your hands. (Don't knead or overwork, as the secret to these cookies is their delicate, sandy texture).

3) Divide the dough into two equal halves. Lay one half on a long sheet of waxed paper and shape into a log along the width of the 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 the logs round store inside an empty paper towel roll.)

4) Preheat the oven to 180C° and line 2 baking sheets with greaseproof paper.

5) Using a sharp, thin knife, slice the logs into 1cm thick rounds. Divide the rounds between the prepared baking sheets, leaving about 2.5cm between them, and refrigerate for 15 minutes.

6) Bake for about 12 to 14 minutes, until the cookies are set on the outside and give off a fragrant, full cocoa aroma. Remove from the oven and set aside to cool for about 5 minutes, on the baking sheets. Transfer the cookies to a wire rack to cool completely and serve when cold.