Preheat your oven to 350°F (175°C). Line a baking sheet with parchment paper.
In a mixing bowl, whisk together the flour, sugar, baking powder, and salt.
In another bowl, beat the eggs, then add the vanilla extract and orange zest.
Gradually add the wet ingredients to the dry ingredients, mixing until combined. Fold in the chopped walnuts.
Divide the dough in half and shape each half into a log about 12 inches long. Place the logs on the prepared baking sheet.
Bake for 25-30 minutes, until the logs are golden brown. Remove from the oven and let cool for 10 minutes.
Using a serrated knife, slice the logs diagonally into ½-inch thick slices. Place the slices cut side down on the baking sheet.
Bake for another 10-15 minutes, until the biscotti are crisp and golden. Let cool completely before serving.