Cocoa powder plus semisweet chocolate chunks provide a double-dose of chocolate in these dunkable, giftable cookies. Martha prepared this recipe in Episode 501 of "Martha Bakes."
Preheat oven to 350 degrees. Line 2 large baking sheets with parchment paper; set aside. In a food processor, pulse the flour, cocoa, baking soda, salt, 1 cup chocolate chunks, and hazelnuts until chocolate and hazelnuts are the size of peas.
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 the mixer on low speed, add the flour mixture and remaining 1 cup chocolate chunks, and beat until just combined.
Turn out the dough onto a generously floured work surface, and divide into four equal pieces. Shape each piece into a 10-by-2-inch log. Transfer to prepared baking sheets. Brush off any excess flour. With the palm of your hand, gently press the logs to flatten slightly. Brush egg white over logs. Sprinkle with sanding sugar, if using.
Bake, rotating halfway through, until logs are just firm to the touch, 20 to 24 minutes. Transfer sheets to a wire rack to cool slightly, about 20 minutes.
Place logs on a cutting board. Using a serrated knife, cut crosswise into 3/4-inch-thick slices. Place wire racks on two large rimmed baking sheets. Arrange slices, cut sides down, on the rack. Bake until biscotti are firm to the touch and completely dry, about 20 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.