Use a food processor to grind the chocolate until it's in very fine bits, almost like powder (see pics).  

Sift together the flour, cocoa and baking soda together onto a piece of parchment paper (or into a bowl) and set aside. Note, I almost always skip sifting but in this case you really don't want lumps in your cocoa powder.  You can save the parchment paper to bake the cookies on.

In a stand mixer (or with an electric mixer), cream the butter, sugar, and salt together until light and fluffy.  Add the egg yolk and vanilla, beating until combined, then scraping down sides.

Finally, add the sifted dry ingredients and grated chocolate, and mix until just combined.  It will be a pretty crumbly dough, just go with it and trust the process, but you may need to give it a few more pulses with your mixer to get it to moisten and come together a bit. 

Lay out some plastic wrap and scrape the dough onto it.  Use your hands to bring it together into one lump (I flatten it with my hands to make the rolling easier).  Wrap it up and chill it in the fridge until just firm, about 20 minutes.  You don't want it to get fully hard, because it gets harder to roll out. 

Preheat your oven to 350 F.  On a very lightly floured surface, roll the dough out gently.  It takes some patience as the dough will still be quite crumbly.  I used little wiggles with gentle pressure, only try to slightly flatten it each roll.  You want them fairly thin so they have a crispness to them, like 1/4-inch thick?

Line baking sheets with parchment paper and cut the dough into desired shapes.  You'll have to re-roll the dough a few times as you cut pieces out, to try and use up as much as possible. Space them apart slightly as they will spread a tiny bit.  If desired, sprinkle decoratively with the raw sugar.

Bake for 8 to 11 minutes, and then leave the cookies on the baking sheet for a few minutes once they come out of the oven before carefully transferring them to cooling racks.  This is because they're fragile until they cool. 

You can store the cookies in an airtight container for up to two weeks, or in the freezer indefinitely.