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, 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.
For the Glaze
Whisk together the icing sugar, and orange juice. Add more orange juice, depending on how thick you'd like the glaze. Whisk in the orange zest, then drizzle over the cooled bread.
Notes
Bread stays fresh in an air-tight container at room temperature or in the fridge for up to 5 days. It also freezes well for up to 3 months. Thaw overnight in the fridge.