1. Preheat oven to 325 degree F and butter 2 large cookie sheets.
2. Whisk together flour, salt, baking powder, and baking soda in a bowl.
3. Beat eggs with zest with an electric mixer on high speed for 2 minutes in another bowl. Slowly add granulated sugar on low speed until combined well. Beat in vanilla. Add flour mixture in 2 batches on low speed until just combined. Stir in dried fruit and nuts until combined.
4. Divide dough into quarters. With floured hands on cookie sheets form each quarter (2 per sheet) into a 9x2-inch log. Bake in upper and lower thirds of oven, switching position of sheets halfway through baking, until pale golden on bottom, about 20 minutes. Keeping oven on, transfer logs on cookie sheets to wire racks to cool for 20 minutes.
5. Carefully transfer a log to a cutting board and with a sharp knife cut biscotti diagonally into 1/3-inch-thick slices, keeping logs together. Brush top of log with some of the egg white and sprinkle with sanding sugar. Transfer pieces a cut side down in one layer onto cookie sheet. Repeat cutting and brushing in same manner with remaining logs.
6. Bake cookies again, in upper and lower thirds of oven, until pale golden on underside, about 10 minutes. Turn cookies and bake until pale golden on underside and crisp, about 10 more minutes. Transfer to wire racks to cool.
7. Biscotti will keep for 3 weeks stored in single layers between sheets of waxed paper in an airtight container. Makes about 5 dozen cookies.