In a medium bowl, whisk together the flour, baking soda, and salt. You want all that salt to be well incorporated!
Using a stand mixer or handheld mixer, beat the butter and sugars together for 1-2 minutes. You are NOT looking for light and fluffy here, you want to combine the butter and sugars only.
Beat in the egg yolks, 2 at a time, followed by the vanilla.
With the mixer on low speed, beat the dry ingredients into the butter mixture until not quite combined. Stop and scrape down the bowl, then give the dough a final stir with the mixer to combine. The dough will be a bit crumbly and quite sticky.
Divide the dough into 3 equal portions. Place each portion on a large piece of plastic wrap or parchment paper and pat it out into a 4-by-6-inch rectangle. Wrap each rectangle separately and refrigerate until chilled, about 30 minutes. This will set the butter and make the dough easier to work with when forming the cookies.
While the cookie dough chills, chop the chocolate into thin shards using a sharp knife and a large cutting board (chopping chocolate on a little board makes a huge mess).
Unwrap one of the chilled dough portions, place it on a parchment paper-covered work surface, and evenly sprinkle the dough with half the chopped chocolate. Top with another unwrapped portion of dough, and again, evenly sprinkle the surface with the remaining chocolate. You should have a towering stack of dough and chocolate. Some chocolate will likely spill out, just poke those chunks back into the tower.
Cover the top of the dough with another piece of parchment paper, then use a large rolling pin to gently roll the tower of dough into a rough rectangle about ¾ inches thick, again tucking any wayward chocolate chunks back into place.
Using a 2.5-inch round cutter (or a thin-rimmed glass), cut out as many rounds of dough as you can (it should be about 10-12 rounds). Place the cut rounds upside down in a single layer (or parchment-separated double layer) on a parchment-lined baking sheet that fits into your freezer (or some other freezer-safe container).
Gently gather the dough scraps together, pat them together lightly, roll out to the same thickness, and cut out as many cookies as you can. You will likely need to do this another time or two.
Freeze the formed cookies for at least 24 hours before baking, or up to two months (in a freezer-safe container).
When ready to bake, preheat the oven to 355 F. Line a large baking sheet with parchment paper and place the cookies on the lined sheet with about 3 inches of room between each round. Do not crowd your pan - these spread quite a lot!
Brush the tops lightly with a little of the beaten egg yolk (if using), being sure that you brush in both directions so you don't wind up with any ugly drips. Lightly sprinkle the tops of each cookie with a little of the fleur de sel. Do not skip this - you need to add what seems like a lot of salt for the chocolate to really pop.
Bake the cookies for 11 minutes 25 seconds. These go from perfectly done to overdone very quickly, so use the light on your oven and start watching them after about 10 minutes.