Whisk 2 tablespoons orange zest into melted butter in a mixing bowl, followed by white sugar, until sugar has dissolved, about 2 minutes. Whisk in eggs until light and fluffy, about 2 more minutes. Stir 1 tablespoon orange juice, milk, and vanilla extract into egg mixture.
Place pistachios into a food processor and pulse until coarsely ground. Add salt, baking powder, and flour to pistachios; pulse until combined, 3 to 4 times. Stir pistachio mixture into wet ingredients until batter is combined. Gently fold in chocolate chips.
Preheat oven to 350 degrees F; generously butter a 9x9-inch square baking dish.
Spread batter into prepared baking dish. Tap the dish several times on a sturdy work surface to eliminate air bubbles.
Bake in the preheated oven until top is lightly golden brown and a toothpick inserted into the center comes out clean, 20 to 25 minutes.
Stir 1 tablespoon orange juice with 2 teaspoons orange zest in a small bowl; mix in confectioners' sugar to form a thin, runny glaze. Drizzle glaze on top of pistachio bars and spread to cover top. Cut into squares once cooled.
