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.

MAKE AHEAD: Cookie dough can be made 1 month ahead; freeze instead of chilling. Slice frozen logs into rounds just before baking.

Divide dough into 4 pieces. Roll each piece into an 8-long log about 1 1/2 in diameter, pushing dough together if it feels crumbly. Wrap tightly in parchment paper and 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 1/4-thick rounds and transfer to 2 parchment-lined baking sheets, spacing 1/2 apart.

Sprinkle cookies with sea salt and bake, rotating baking sheets halfway through, until set around edges and centers look dry, 10-12 minutes. Transfer to wire racks and let cool.