In a medium bowl, sift together the flour and baking powder. Add salt and set aside.
Using an electric mixer or stand mixer, beat the butter, sugar, orange zest and vanilla until light and fluffy, about 1 minute. Add 1 egg at a time and beat until each is mixed in.
Reduce speed to low and slowly add flour and mix until just combined. Fold in pistachios. Cover and chill for in fridge for 1 hour.
Preheat oven to 350°F. Line baking sheet with parchment paper. Divide dough in half and place on baking sheet. Shape dough into two 11x2 inch or 8x4 inch logs, keeping at least 3 inches between them. Brush each log with egg white and sprinkle 1 tsp sugar over the top and sides of each.
Bake for 30 minutes or until the biscotti is light brown and firm to the touch. Remove from oven and let cool slightly for 10 minutes.
Transfer logs to cutting board and cut on the diagonal 1" thick slices with a sharp serrated knife. (If it's crumbly, let cool for longer) Arrange the slices on the baking sheet and bake for 20 minutes, flipping the biscotti halfway through. Transfer to a wire rack and allow to cool.
If dipping in dark chocolate, place chocolate in heatproof bowl and microwave in 30 second intervals until chocolate is smooth. Dip biscotti and transfer back to wire rack to cool.