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1
Preheat the oven to 350F.
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2
Line a large baking sheet with parchment paper; set aside.
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3
In a food processor, pulse the flour, cocoa, baking soda, salt, 1 cup chocolate chunks, and hazelnuts until chips and hazelnuts are the size of peas.
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4
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.
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5
Switch to the paddle attachment.
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6
With mixer on low speed, add the flour mixture and remaining 1 cup chocolate chunks, and beat until just combined.
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7
Turn out the dough onto a lightly floured work surface, and divide into three equal pieces.
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8
Shape each piece into an 18-inch log.
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9
Transfer to prepared baking sheet.
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10
With the palm of your hand, gently press the logs to flatten slightly.
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11
Brush egg white over logs.
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12
Sprinkle with sanding sugar, if using.
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13
Bake, rotating pan halfway through, until logs are just firm to the touch, 20 to 24 minutes.
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14
Transfer sheet to a wire rack to cool slightly, about 20 minutes.
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15
Place logs on a cutting board.
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16
Using a serrated knife, cut 3/4-inch-thick slices on the diagonal.
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17
Place a wire rack on a large rimmed baking sheet.
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18
Arrange slices, cut sides down, on the rack.
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19
Bake until biscotti are firm to the touch and completely dry, 10 to 12 minutes.
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20
Remove pan from oven; let biscotti cool completely on the rack.
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21
Biscotti can be kept in an airtight container at room temperature for up to 1 week.
