Preheat oven to 325 degrees. Butter and flour 2 baking sheets, knocking out excess flour.
In a bowl of a stand mixer fitted with a paddle attachemt (or you can use a hand-held mixer like I do), beat together the flour, brown sugar, espresso powder, zest, baking soda, baking powder, and salt until well combined. Beat in eggs and vanilla just until a dough forms. Stir in hazelnuts.
On a lightly floured surface, knead dough several times. Halve dough with floured hands and form each half into a flattish log, 13" long by 1" wide. Put both logs at least 4" apart on one baking sheet and brush with egg wash.
Bake logs in the center of the oven for 30 minutes. Cool on baking sheet set over a rack for 10 minutes.
Transfer logs, one at a time, to a cutting board. Use a serrated knife to slice the logs on a crosswise diagonal into 3/4-inch-thick slices.
Arrange biscotti, cut sides down, on 2 baking sheets.
Bake biscotti in upper and lower thirds of oven for 8 minutes. Remove sheets from oven and turn biscotti over with a spatula. Return biscotti to oven, switching positions of sheets, and bake 8 minutes more, or until biscotti are slightly darker. Transfer biscotti to racks and cool.
Biscotti keep 3 days in an airtight container at room temperature.