I just made this recipe once, and I substituted toasted sliced almonds for
the hazelnuts. It was really good. Preheat oven to 350-F. Spread the
hazelnuts on a baking sheet and toast in the oven for 8 to 10 minutes,
until fragrant. Place the nuts in a dish towel and rub them together to
remove some of the skin. Coarsely chop the hazelnuts and set aside. In a
large bowl, using a hand-held electric mixer, beat the butter at medium
speed until soft and creamy. Add the sugar and lemon zest and beat until
the mixture is very light and fluffy, about 2 minutes. Beat in the eggs,
one at a time, and then beat in the vanilla. In a medium bowl, toss the
flour, baking powder and salt. Gradually add the flour mixture to the
butter, beating on low speed until a smooth dough forms. Using a wooden
spoon, work in the hazelnuts until evenly distributed. Divide the dough in
half in the bowl. With lightly floured hands, form the dough into two
10-by-2 inch rectangular logs. Place the logs on an ungreased baking sheet,
at least 2 inches apart. Bake for 30 minutes, or until the logs are set and
golden brown. Remove from the oven and let cool on the baking sheet for 15
minutes. Using a serrated knife and a sawing motion, carefully cut the logs
into 1/2-inch diagonal slices. discard the end pieces. Arrange the slices,
cut sides down and close together, on 2 baking sheets. Bake the biscotti on
the top and middle racks of the oven for 8 to 9 minutes, or until lightly
browned. Turn the biscotti over, switch the position of the baking sheets
and continue baking until lightly browned on the other side, about 9
minutes longer. Let the biscotti cool completely on the baking sheets.