Rub Orange Zest Into Sugar. Add granulated sugar and orange zest to the bowl of a stand mixer. Use clean hands to rub the orange zest into the sugar, until it's fragrant, and the sugar is tinted orange. (This distributes the orange essential oils into the sugar, which gives the cookies a more vibrant citrus flavor.!)
Cream Butter & Sugars. Add brown sugar and butter cubes to the mixer and cream with the paddle attachment on medium speed 2-3 minutes, or until smooth and well blended. Add vanilla and stir to combine.
Add Dry Ingredients. Scrape down the sides of the bowl and add gluten-free flour and salt. Mix on low speed until no large patches of flour remain. (The dough will look fairly dry and crumbly at this point--this is normal!)
Mix in Chocolate. Add chocolate chunks and mix on LOW speed until just evenly incorporated. The dough will still look crumbly, but should hold together when pinched between your fingers.
Shape & Chill Dough. Divide the dough in half and shape each half into a roughly 6-inch log. It's okay to really pack it together! Wrap each log in plastic wrap and chill in the refrigerator at LEAST 2 hours, up to 3-4 days. DON'T skip the chill time, or your cookies will not slice nicely & will spread too much in the oven.
Preheat & Prep. When you're ready to bake, preheat the oven temperature to 350 degrees F. Line a baking sheet with parchment paper.
Slice & Prep Dough. Remove one log of dough from the fridge and use a sharp serrated knife to slice into 12 slices. If you hit a large chocolate chunk that makes slicing difficult, use a gentle sawing motion to work your way through it. If the dough happens to crack, it's 100% fine to just pinch it back together. The dough is pretty forgiving
Bake & Cool Cookies. Place 12 cookie slices on prepared baking sheet and bake in preheated oven 11-13 minutes, or until set. (The cookies will not brown much, if at all!) Allow cookies to cool 1-2 minutes on baking sheet before transferring to a wire rack to cool completely. Repeat this process with the second log of dough for another 12 cookies.
Serve & Store. Enjoy right away or store in an airtight container at room temperature 2-3 days. Or, freeze baked cookies in a freezer bag up to 2 months!