Preheat oven to 350 degrees F. Line a baking sheet with parchment paper.
In an electric mixer, using whisk attachment on medium-high speed, whisk eggs, 1/2 cup sugar, milk, vanilla, and orange zest for 3 minutes.
Add flour, baking powder, baking soda, and salt. With the paddle attachment, beat on low speed just until combined. Fold in pistachios.
Place some water on a clean surface. Flour your hands. The dough will be sticky, but the water and flour will keep it from sticking. Shape the dough into one large log, about 5 inches wide and 8-10 inches long. The biscotti log will puff up when baking, so don't worry if it doesn't look like much. Brush with egg white and sprinkle with extra 2 tablespoons sugar.
Bake 20-30 minutes or until firm to touch. Remove from oven and let cool completely. You can slice and finish the biscotti now, but you will get cleaner cuts if you refrigerate overnight.
When ready to finish baking, set oven to 300 degrees F. Slice biscotti about 1/2-inch to 3/4-inch thick. I think the thinner, the better. The thinner they are, the more toasted the pistachios will get in the oven. Arrange on a baking sheet and bake 5-10 minutes per side until golden.