2½ c all-purpose flour
½ c unsweetened cocoa powder
¾ t kosher salt
½ t baking soda
1½ c (2½ sticks) unsalted butter, room temperature
1½ c (lightly packed) light brown sugar
1 t vanilla extract
1 large egg white
5 oz bittersweet or semisweet chocolate, chopped
1 c unsalted, shelled raw pistachios, coarsely chopped
Flaky sea salt (such as Maldon)
Whisk flour, cocoa powder, kosher salt, and baking soda in a medium bowl. Using an electric mixer on high speed, beat butter, brown sugar, and vanilla until light and fluffy, about 4 minutes. Reduce speed to low and gradually add dry ingredients; mix just to combine, then mix in egg white. Fold in chocolate and pistachios.
Divide dough into 4 pieces. Place each piece in the center of a sheet of parchment paper and form into a log, about 8″ long and 1½” in diameter. Just like you’d roll up a poster, wrap the parchment around the log, smoothing the paper and pressing out air with your hands. Twist the ends of the parchment tightly to compact the dough log. After wrapping, roll log on a flat surface to create a tight cylinder. Let chill until firm, at least 4 hours. (The colder your dough, the easier it will be to slice.)
Place racks in lower and upper thirds of oven; preheat to 350°F. Working with 1 log of dough at a time and using a serrated knife, cut logs into ¼”-thick rounds and transfer to 2 parchment-lined baking sheets, spacing ½” apart.
Sprinkle cookies with sea salt and bake, rotating baking sheets halfway through, until set around edges and centers look dry, 10–12 minutes. Do not overbake. Transfer to wire racks and let cool.
Note: Cookie dough can be made 1 month ahead; make parchment logs and wrap in plastic before freezing. Slice frozen logs into rounds just before baking.