In the bowl of an electric mixer, fitted with a paddle attachment, blend the flour, the sugar, the baking soda and the salt until combined well.
In a small bowl whisk together the whole eggs, the yolk, the vanilla and the zest, add the mixture to the flour mixture, beating until a dough is formed and stir in the almonds.
Turn the dough out onto a lightly floured surface, knead it several times and halve it.
Working on a large buttered and floured baking sheet, with floured hands form each piece of dough into a flattish log 30 cm long and 5 cm wide, arrange the logs at least 8 cm apart on the sheet, and brush them with the egg wash.
Bake the logs in the middle of a preheated 150ºC oven for 50 minutes and them cool on the baking rack for 10 minutes.
On a cutting board, cut the logs crosswise on the diagonal into 1 cm thick slices, arrange the biscotti, cut sides down, on the baking sheet and bake them, in the 150ºC oven for 15 minutes on each side.
Note: it may take a bit longer in the oven to dry the biscotti.
Transfer the biscotti to racks to cool and store them in airtight containers.
MAKES: about 48 biscotti