Step 1: In a large mixing bowl, cream together the softened unsalted butter, granulated sugar, and orange zest until light and fluffy, about 2-3 minutes.
Step 2: In a separate bowl, whisk together the all-purpose flour and salt. Gradually add the dry ingredients to the wet ingredients, mixing on low speed until just combined. Be careful not to overmix.
Step 3: Gently fold in the dark chocolate chips until evenly distributed throughout the dough.
Step 4: Gather the dough into a ball, flatten it into a disc, wrap it tightly in plastic wrap, and refrigerate for at least 30 minutes (or up to 2 hours) to allow the butter to firm up.
Step 5: Preheat your oven to 325°F (160°C). Line a baking sheet with parchment paper.
Step 6: On a lightly floured surface, roll out the chilled dough to approximately 1/4-inch thickness. Use a 2-inch round or fluted cookie cutter to cut out shapes. Reroll scraps as needed.
Step 7: Place the cut-out cookies on the prepared baking sheet, leaving about 1 inch between each cookie. You may need to bake in batches.
Step 8: Bake for 15-20 minutes, or until the edges are lightly golden and the centers are still pale. Do not let them brown too much.
Step 9: Remove from the oven and let the cookies cool on the baking sheet for 5 minutes before carefully transferring them to a wire rack to cool completely. These chocolate orange shortbread cookies are delicate when warm.
Step 10: Store cooled chocolate orange shortbread cookies in an airtight container at room temperature for up to 5 days, or freeze for longer storage. (For reliable storage, I highly recommend using quality Basics Glass Food Storage containers.)