1. Preheat the oven to 350°F. Line a large baking sheet with parchment paper; set aside. In a food processor, pulse the flour, cocoa, baking soda, salt, 1 cup chocolate chunks, and pistachios until chips and pistachios are the size of peas.
2. In the bowl of an electric mixer fitted with the whisk attachment, beat the whole eggs and granulated sugar until the mixture holds a ribbon-like trail on the surface for a few seconds when you raise the whisk. Switch to the paddle attachment. With mixer on low speed, add the flour mixture and remaining 1 cup chocolate chunks, and beat until just combined.
3. Turn out the dough onto a lightly floured work surface, and divide into three equal pieces. Shape each piece into an 18-inch log. Transfer to prepared baking sheet. With the palm of your hand, gently press the logs to flatten slightly. Brush egg white over logs. Sprinkle with sanding sugar, if using.
4. Bake, rotating pan halfway through, until logs are just firm to the touch, 20 to 24 minutes. Transfer sheet to a wire rack to cool slightly, about 20 minutes.
5. Place logs on a cutting board. Using a serrated knife, cut ¾-inch-thick slices on the diagonal. Place a wire rack on a large rimmed baking sheet. Arrange slices, cut sides down, on the rack. Bake until biscotti are firm to the touch and completely dry, 10 to 12 minutes. Remove pan from oven; let biscotti cool completely on the rack. Biscotti can be kept in an airtight container at room temperature for up to 1 week.