1 cup shelled unsalted pistachios (about 5 ounces)
Line a large heavy baking sheet with parchment paper. In a large bowl, using a wooden spoon, beat the butter with the sugars, vanilla and orange zest until creamy. Beat in the egg. Mix in the flour, almonds, baking powder, cinnamon and alt. Stir in the pistachios.
Form the dough into two 12-inch logs and wrap in wax paper. Place on a small baking sheet and refrigerate until chilled, about 2 hours.
Preheat the oven to 325°. Unwrap the logs and set them on the prepared baking sheet at least 3 inches apart. Bake for about 35 minutes, or until golden and dry to the touch. Let cool for about 30 minutes, then cut the logs on the diagonal into 3/4-inch-thick slices. Line the baking sheet with fresh parchment and arrange the biscotti on the paper without touching each other. Bake for about 25 minutes, or until golden. Cool on a rack.
The logs can be refrigerated overnight or wrapped in foil and frozen for up to 1 month. Thaw before baking. The baked biscotti can be stored in a covered cookie tin for up to 5 days.