Lightly grease a 9-inch square pan with nonstick cooking spray and/or line with a sling of parchment paper.

In a large bowl or the bowl of a food processor, rub or pulse the granulated sugar and orange zest together until the sugar is a pale orange color. Add the flour and salt and mix or blitz to combine.

Cut the butter into pats and add to the flour mixture. Cut the butter in until the mixture resembles fine crumbs and it holds together in a ball when squeezed in your hand.

Transfer the dough to the prepared pan and press into an even layer. Cover and transfer to the refrigerator to chill for at least 30 minutes and up to 1 day.

Preheat your oven to 350 degrees F.

Bake the pan of shortbread for 15 minutes. Remove from the oven and, using the tines of a fork, poke 8 rows of holes into the dough. Using a bench scraper or knife, score all the way through the dough between each row. Cut the rows horizontally in half or in thirds so that you have 16 to 24 bars. Return to the oven and continue to bake until lightly golden brown, about another 15 minutes. Set aside to cool completely.

Remove the shortbread from the pan, carefully break the bars apart along the scored lines and set on a rack-lined baking sheet.

Melt the chocolate in the microwave or over a pan of simmering water, then use a spoon to splatter and drizzle the chocolate over the bars. Scatter with turbinado sugar, if using, and allow the chocolate to cool and set before serving or transferring to a container to store at room temperature for up to 2 weeks or in the freezer for 3 months.