Heat the oven to 350°F (175°C) and line a baking sheet with parchment paper. Whisk together the flour, sugar, baking powder, and salt in a large bowl.

In a separate bowl, whisk the melted, slightly cooled butter with the two whole eggs, egg yolk, vanilla, and orange zest. Add the wet mixture to the dry ingredients and stir just until a soft dough forms.

Fold in the chopped pistachios and dried cranberries until evenly distributed. Divide the dough in half and shape each portion into a 10- to 12-inch-long, 2- to 3-inch-wide log on the prepared baking sheet.

Flatten the tops slightly and brush the logs with the reserved egg white. Bake for 25 to 30 minutes, until the logs are firm and lightly golden.

Cool the baked logs for 15 to 30 minutes so they firm up without becoming completely cold. Use a sharp serrated knife and a gentle sawing motion to cut them diagonally into slices about 1/2 to 3/4 inch thick.

Arrange the slices cut-side down on the baking sheet and bake again at 325°F to 350°F (165°C to 175°C) for 8 to 10 minutes per side. Turn the slices once and continue baking until they are dry and crisp.

Transfer the biscotti to a wire rack and cool completely. They become firmer as they cool, so wait until they are fully cooled before storing or dipping in optional white chocolate.