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 (fitted with the paddle attachment if using a stand mixer), beat at medium-high speed until the mixture is pale in color, about 2 minutes, periodically scraping down the sides with a rubber spatula (don't skip this, it makes a difference). Add egg yolk and blend until combined.
Add flour mixture and mix on low until combined. If there are dried bits of flour left around the bowl, use a rubber spatula to smoosh them in with the dough. Add orange peel, pistachios and chopped chocolate, fold in with a rubber spatula. (The dough will be crumbly (although with the modifications, less so). If necessary, use the electric mixer to add the mix-ins, or work them in with your hands until fully incorporated.)
To form the cookies, line an 8 by 8-inch baking pan with plastic wrap and leave a generous amount of overhang on all sides. Transfer the dough to the prepared pan and press firmly to flatten in an even layer. Cover with plastic wrap and chill dough for 8 hours or overnight before baking.
Preheat the oven to 350 degrees. Line 2 baking sheets with parchment paper.
Using the plastic wrap overhang, remove the dough from the pan and cut the square into three equal rectangles. It's going to be tough to get out of the pan but just be patient and pull gently on the plastic to gradually remove from pan.
Cut each rectangle crosswise into roughly 1⁄3-inch-thick slices and lay them flat on the prepared baking sheets, about 3⁄4-inch apart. (If the dough crumbles when slicing, simply push the mixture together to reform the cookie.) Bake until lightly golden on the bottom but still blonde on the edges, 15 to 17 minutes.
Remove from the oven, allow cookies to cool on the sheets. The cookies will keep in an airtight container at room temperature for up to 1 week.