Actually listed as "Intensely Chocolate Sables" but I don't trust myself to remember to look under the "I". If you thought Chocolate Skinnies were chocolate-y, you ain't seen nothin' yet :-)
from https://smittenkitchen.com/2013/01/intensely-chocolate-sables/
who said "Inspired by Balthazar, adapted quite a bit from Miette"
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*
1/4 teaspoon baking soda
1/2 cup (1 stick, 4 ounces or 115 grams) unsalted butter, at room temperature
½ c (100 g) granulated sugar (I used about 1/3 c. sugar and rest splenda)
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
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. 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.
*The author STRONGLY recommends using Dutched cocoa (i.e. cocoa with alkali added) to enhance depth of chocolate flavor, depth of color, and structural integrity. Considering how delicate my dough was with Dutched cocoa, I wouldn't want to risk it with regular cocoa.