• In the bowl of a stand mixer fitted with the paddle attachment (or using a hand mixer), beat the butter on medium speed for about 2 minutes, until smooth and creamy in texture.
• With the mixer running on low speed, add the sifted powdered sugar one tablespoon at a time. Once all of the sugar has been added, continue mixing on medium speed for 2 to 3 minutes, until the mixture is light and fluffy.
• Add the vanilla and mix until incorporated.
• Add the ground pistachios and mix until evenly combined. Be sure to scrape down the sides and bottom of the bowl after each addition to ensure everything is mixing evenly.
• Add the orange juice and the orange zest, then mix until incorporated.
• Add the all-purpose flour and the salt. Fold the ingredients together with a spatula, or mix on the lowest speed for about 30 seconds, just until no dry streaks of flour remain. Avoid overmixing.
• The dough will be very soft, and that's exactly how it should be. Shape it into a disk, lightly dust both sides with powdered sugar, wrap it tightly in plastic wrap, and refrigerate for 2 to 4 hours or overnight.
• Don't skip the chilling time. The dough must be completely cold before rolling it out. To check, gently press the center of the dough with your finger. If it still feels soft, return it to the refrigerator until it is thoroughly chilled.
• Once chilled, divide the dough in half. Keep one half wrapped in the refrigerator while you work with the other.
• On a lightly floured surface, roll the dough to 1/4-inch thick. If the dough feels too firm straight from the refrigerator, let it sit at room temperature for 2 to 5 minutes before rolling.
• Cut the dough into your desired shapes, such as circles, stars, or rectangles. Transfer the cookies to a baking sheet lined with parchment paper or a silicone baking mat.
• Place the baking sheet in the freezer while the oven preheats. This step is important because rolling the dough softens the butter. Chilling the shaped cookies again ensures the butter is cold and firm before baking, helping the cookies hold their shape and preventing excessive spreading.

To bake

• Preheat the oven to 350°F.
• Bake for 10 to 20 minutes, depending on the size of the cookies. Smaller cookies will bake more quickly, while larger cookies will need additional time.
• If your cookies are larger than about 1 1/2 inches, gently prick the tops once or twice with a fork before baking. This allows steam to escape and helps prevent large air pockets from forming, giving the cookies a smoother surface.
• The cookies are done when the centers are still pale blonde and the edges are lightly golden. This gives them the perfect balance of buttery texture and crisp edges. Avoid overbaking, as shortbread cookies will become dry and tough.
• Let the cookies cool completely.

To decorate

• Meanwhile, melt the chocolate over a double boiler. Dip one end of each cookie into the melted chocolate, letting any excess drip back into the bowl. For a cleaner finish, gently scrape the underside of the dipped cookie against the edge of the bowl or with a small offset spatula.
• Immediately sprinkle the finely ground pistachios over the chocolate and place the cookies on a wire rack until the chocolate has completely set.