Preheat the oven to 180C/350F/Gas 4. Line the bottom of an 9x5" loaf pan with baking paper, and grease the sides. Set aside.
Whisk together the flour, sugar, baking powder, baking soda, salt orange zest, and chocolate chips.
Whisk together the egg, orange juice, buttermilk, vanilla extract and oil.
Pour the wet ingredients into the dry ingredients, and mix until just combined. Do not over-mix.
Pour the batter into the prepared pan, sprinkle the extra chocolate chips on top, and bake for 50 - 60 minutes, or until a toothpick inserted into the centre comes out clean.
Cover loosely with aluminium foil halfway through baking to prevent the bread from browning too much. Allow to cool completely in the pan on a wire rack before removing and glazing.
Notes
I think it will be best if it's baked in two tins instead of one. It took me almost 1 hr 20 mins to get this cake done. The cake can be stored at room temp for 2-3 days in an airtight container, unless you are living in an extremely humid place.