A stand mixer is useful for this recipe but a large bowl and hand mixer is fine too. Place the butter in the bowl of your mixer fitted with the paddle attachment. Beat on medium speed for a minute or two, until light and creamy. Sift in the powdered sugar and add the orange zest. Beat again for about a minute until fluffy and pale.
Now add the flour and salt. Beat on low speed until the flour is just incorporated, then add the chocolate chips. Beat briefly to combine them into the soft, slightly sticky dough.
Tip the dough out into an 8-inch square baking pan lined with non-stick baking paper, leaving an overhang for easy removal.
Use lightly floured fingertips or the base of a measuring pan to evenly and firmly press the dough into the pan. Refrigerate for 1 hour, until firm.
In the meantime preheat the oven to 150 C.
Once chilled, remove the dough from the pan using the paper overhang. Slice into 24 biscuits, by first using a sharp knife to cut the dough into 3 equal columns (about 2.5 inches wide). Then in the other direction, cut the dough into 8 bars, about 1 inch wide. (see photos below)
Place the dough back in the pan. Prick the top of each biscuit with a fork for the signature packaged shortbread biscuit look. Bake for 20 minutes.
Take the half-baked dough out of the oven. It will be puffy and the cut marks will be a bit less visible now (see last photo below), but using them as a guide, cut the dough the same way again, as well as use the fork to prick holes again if they've filled up.
Bake again for 20 to 25 minutes, until the edges are golden brown but the surface is still pale and slightly puffy. I found baking till the tops were brown as well led to harder, crunchier biscuits. Not a bad thing, but not the ideal texture of a classic shortbread biscuit.
While the biscuits are still hot and in the pan, slice them one final time along the same lines. Then let them cool for 30 minutes at room temperature.
Lift out the biscuits using the paper overhang again and set on a wire rack to cool completely. The biscuits should come apart very easily now.
Store in an airtight tin at room temperature for a week or so and they can be frozen for weeks. The flavour and texture gets better over time! Happy baking :)