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1
Put oven rack in middle position and preheat oven to 350u00b0F. Line a large baking sheet with parchment paper.
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2
Toast hazelnuts in a shallow baking pan until lightly colored and skins are blistered, 10 to 15 minutes. (Leave oven on.) Wrap nuts in a kitchen towel and let steam 1 minute, then rub off any loose skins in towel while nuts are still warm (don't worry about skins that don't come off). Cool nuts completely, then very coarsely chop.
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3
Pulse together sugar and 1/2 cup chopped hazelnuts in a food processor until finely ground. Transfer to bowl of mixer along with flour and beat at medium speed until combined well. Add eggs and vanilla and beat just until a dough forms. Reduce speed to low, then add remaining chopped hazelnuts and mix until incorporated. Turn off mixer and knead in any loose hazelnuts with your hands.
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4
Halve dough and, with dampened hands, form each half into a roughly 10- by 2- by 1-inch log on lined baking sheet, arranging logs 3 inches apart.
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5
Bake until golden and set but still soft to the touch, 25 to 30 minutes. (Leave oven on.) Cool logs on baking sheet on a rack 10 minutes.
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6
Transfer logs to a cutting board, discarding parchment, and cut logs with a serrated knife on a slight diagonal into 1/2-inch-thick slices.
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7
Arrange slices, cut sides down, in 1 layer on unlined baking sheet. Bake slices, turning over once, until golden and crisp, 20 to 25 minutes more. Cool biscotti completely on sheet on rack, about 30 minutes.
