Intensely Chocolate Sables

https://smittenkitchen.com/2013/01/intensely-chocolate-sables/

Makes 40 to 48 2-inch thin cookies, fewer if thicker
1 cup (125 grams) all-purpose flour
1/3 cup (30 grams) Dutched cocoa powder (see Updated Note)
1/4 teaspoon baking soda
1/2 cup (1 stick, 4 ounces or 115 grams) unsalted butter, at room temperature
1/2 to 2/3 cup (100 to 135 grams) granulated sugar (less for a more bittersweet cookie)
1/4 teaspoon fine sea salt
1 large egg yolk
1/2 teaspoon vanilla extract
3 1/2 ounces (100 grams) semi- or bittersweet chocolate, grated or finely chopped until almost powdery in a food processor
Coarse sugar (turbinato/sugar in the raw or decorative) for sprinkling
Sift together the flour, cocoa and baking soda together onto a piece of waxed paper or into a bowl and set aside. (I almost always skimp on sifting wherever possible, but my cocoa is always lumpy, so this is unavoidable.)
Cream butter, sugar and salt together in a large bowl with an electric mixer until light and fluffy. Add egg yolk and vanilla, beating until combined, then scraping down sides. Add dry ingredients and grated chocolate together and mix until just combined.
Scrape dough onto a piece of plastic wrap, wrap it up and chill it in the fridge until just firm, about 30 to 45 minutes. No need to get it fully hard, or it will be harder to roll out. Dough can be refrigerated until needed, up to a two days, or frozen longer, but let it warm up and soften a bit before rolling it out for decreased frustration.
Heat oven to 350 degrees. On a floured surface, roll dough gently — it will still be on the crumbly side, so only attempt to flatten it slightly with each roll — until it is 1/8-inch thick (for thin cookies, what I used), 1/4-inch thick (for thicker ones) or somewhere in-between (I suspect the Balthazar ones are rolled to 3/8-inch). Cut into desired shapes and space them an inch apart on a parchment-lined baking sheet. Sprinkle decoratively with coarse sugar. Bake for 8 to 10 minutes (for thinner cookies) or 10 to 12 minutes (for thicker ones). Leave cookies on baking sheets out of the oven for a couple minutes before gently, carefully transferring them to cooling racks, as they’ll be fragile until they cool.