Whisk together flour and salt in a medium bowl. Combine butter and sugar in a large bowl, or in the bowl of a stand mixer. Using an electric mixer, beat at medium-high speed until the mixture is pale in color, about 2 minutes. Add egg yolk and blend until combined.
Add the flour mixture and mix on low until combined. Use a rubber spatula to incorporate any dry bits. Fold in orange peel, pistachios, and chopped chocolate. (The dough will be crumbly. Use your hands if needed to fully incorporate.)
Line an 8-by-8-inch baking pan with plastic wrap, leaving overhang on all sides. Transfer the dough to the prepared pan and press firmly into an even layer. Cover with plastic wrap and chill for at least 8 hours or overnight.
Preheat oven to 350°F (177°C). Line 2 baking sheets with parchment paper.
Using the plastic wrap overhang, remove the dough from the pan. Cut the square into three equal rectangles. Slice each rectangle crosswise into 1/3-inch-thick slices (about 48 cookies). Arrange them on the baking sheets, spaced 3/4-inch apart. Bake for 15-17 minutes, until lightly golden on the bottom.
Let cookies cool completely on the baking sheets. Store in an airtight container at room temperature for up to 1 week.
Tips
Use a spoon to fluff and scoop flour into your measuring cup for accuracy.
Chilling the dough is essential to ensure it holds together when slicing and baking.
For added flavor, toast the pistachios lightly before mixing them in.