The accent of the orange zest is pleasant in this classic loaf, but you can use lemon zest in place of it for an equally delicious accent.
Preheat the oven to 350 F. Grease a 9-x-5-inch and line the pan with parchment paper so that it comes up the sides.
Whisk the oil, eggs, sugar, zest and vanilla together in a large bowl.
In a separate bowl, sift the flour, baking powder, baking soda and salt. Stir in the zucchini to coat it with the flour and add this to the wet mixture, stirring until blended. Stir in the chocolate chips (if using) and scrape the batter into the prepared pan.
Bake the loaf for 50 to 60 minutes, until a tester inserted in the centre of the cake comes out clean. Cool the cake completely in the pan until turning out to slice.
The loaf will keep, well wrapped, for up to 3 days, and can be frozen for up to 3 months.