Sift flour, cocoa powder, baking soda and salt into a medium bowl and whisk to blend evenly.

In the bowl of an electric stand mixer fitted with the paddle attachment, beat butter and sugar on medium speed for 4-5 minutes, until very pale and fluffy. Stop the mixer and scrape down the sides of the bowl as necessary. Reduce the speed to low and mix in egg yolk and vanilla extract until combined. Add the flour mixture and mix just until blended, then add chopped chocolate and continue mixing until the dry ingredients are evenly incorporated. The dough will be stiff.

Turn the dough out onto a large piece of plastic wrap and form into a log about 10 inches long, rolling it back and forth to form a smooth cylinder. Wrap well in the plastic wrap and refrigerate for at least 2 hours, until firm.

Preheat your oven to 350°F and line two large baking trays with parchment paper.

Use a sharp knife to cut the log into slices with just over ¼-inch thickness. Place the rounds 2 inches apart onto prepared baking trays and sprinkle a few flakes of sea salt over top. Bake for 10-12 minutes, until the tops begin to crack and the edges feel dry. Let cookies cool for 5 minutes on trays before transferring them individually to a wire rack to finish cooling.