Preheat the oven to 350°F (175°C). Line a large baking sheet with parchment paper.

Whisk the flour, sugar, baking powder, and salt in a large bowl. In a small bowl, beat the eggs with the vanilla and optional orange zest.

Pour the egg mixture into the flour mixture. Stir until a thick, slightly sticky dough forms, then fold in the chopped almonds. If dry patches remain, knead the dough gently in the bowl a few times just until combined.

Transfer the dough to the prepared baking sheet. With lightly dampened hands, shape it into a log about 12 inches long and 3 inches wide (30 x 8 cm).

Bake until the log is lightly golden and firm around the edges, about 25 minutes. Place the baking sheet on a rack and let the log cool for 15 minutes. Reduce the oven temperature to 325°F (165°C).

Using a serrated knife and a gentle sawing motion, cut the warm log diagonally into slices about 1/2 inch thick (1.25 cm). Arrange the slices cut-side down on the baking sheet.

Bake for 10 minutes, turn each biscotti over, and bake for 10 minutes more, until dry and golden at the edges. For extra-crisp biscotti, leave them in the switched-off oven with the door slightly ajar for 5 minutes.

Cool the biscotti completely on a wire rack. They will become crisper as they cool.