(from the now-closed Korova restaurant in Paris but still available at the Pierre Hermes patisserie)
Makes 2 dozen (recipe can be doubled)
1 1/3 c flour
1/3 c Dutch process cocoa, unsweetened
1 t baking soda
1 stick + 3 T unsalted butter (11 T in all), at room temperature
1/2 c light brown sugar
1/4 c white sugar
1 1/4 t fine sea salt (Fleur De sel)
1/2 t vanilla extract
5 oz bittersweet chocolate, chopped into 1/4-inch pieces (1 cup)
In a medium bowl, sift flour with cocoa and baking soda and set aside.
In a large bowl, cream together butter, brown sugar, white sugar, sea salt and vanilla. Beat by hand until combined. Beat in sifted dry ingredients until just blended. Dough will be crumbly but will hold together. Knead in chocolate pieces until evenly distributed.
Form a log approx 1 1/2 inches in diameter. Wrap in plastic and refrigerate until firm, at least 2 hours. (can be refrigerated for up to 3 days or frozen for a month, if tightly wrapped)
Preheat oven to 350*. Line 2 baking sheets with parchment paper.
Using a sharp knife, cut 3/8 inch slices from the log and arrange about 1 inch apart on the baking sheets. If slices crumble, reform the cookies by pressing the dough together.
Bake the cookies on the lower and middle racks until puffed and cracked on top, about 17 minutes. Shift pans from top to bottom and front to back about halfway through (or use the fan if you have a convection oven). Let cookies cool on the sheets for 5 minutes before transferring to a wire rack to cool completely.