In a medium bowl, stir together the flour, cocoa powder, espresso powder, and baking soda. Set aside.
In a mixing bowl, cream together the softened butter, sugar, egg yolk, and vanilla extract with an electric mixer until creamy. Add in the flour mixture and mix on low speed until it becomes crumbly. Mix in the chopped chocolate until distributed.
Pour the dough out onto a sheet of plastic wrap and press the crumbs together with your hands until it sticks together. Divide in half and form each half into a log shape about 2" wide and 6" in length. Place each log onto a separate sheet of plastic wrap. Fold the wrap over the log and roll against the counter to create a cylindrical shape. Flatten the ends of each log. Refrigerate for at least 4 hours or overnight until solid.
Preheat oven to 350F and line a baking sheet with parchment paper.
Unwrap the chilled logs and use a serrated knife to cut ½" discs out. This dough is very crumbly so the cookies may break apart while cutting but you can just press them back together.
Place cookie rounds on the baking sheet with 1-2" between each cookie and bake for about 15 mins.
Let the cookies cool on the baking sheet for 5-10 minutes before moving them to a wire rack to cool completely.
Melt 30g of chocolate and transfer to a small piping bag or ziploc bag with the tip cut off. Drizzle the chocolate across each cookie and sprinkle with flaky salt before it hardens.