Preheat your oven to 350°F (175°C) and line two baking sheets with parchment paper.
In a medium bowl, whisk together the flour, baking powder, and salt until well combined. Set aside.
In a large mixing bowl, use a mixer to cream together the unsalted butter and granulated sugar until light and fluffy, about 2-3 minutes.
Beat in the large egg, vanilla extract, and the first addition of orange zest into the butter mixture until fully incorporated.
Gradually add the flour mixture to the wet ingredients, mixing on low speed until just combined. Be careful not to overmix!
Gently fold in the chopped pistachios, ensuring they are evenly distributed throughout the dough.
Using a cookie scoop or tablespoon, drop rounded scoops of dough onto the prepared baking sheets, leaving space between each cookie.
Bake in the preheated oven for 10-12 minutes, or until the edges are lightly golden. The centers may look slightly underbaked, but they will continue to firm up as they cool.
While the cookies are baking, prepare the glaze by whisking together the powdered sugar, freshly squeezed orange juice, and the second addition of orange zest until smooth.
Allow the cookies to cool on the baking sheets for a few minutes before transferring them to a cooling rack. Once cooled, drizzle the orange glaze over the cookies for a beautiful finish.