Thoroughly wash the skin of the orange and remove the stem, if it's still on. Add around 4 cups of water to a small saucepan and place the whole orange in it. Over medium heat, simmer the orange for 30 minutes until you can pierce the skin with no resistance. Drain the water and allow the orange to cool.
Preheat the oven to 170°C (340°F). Grease or line a 20 cm (8 inch cake tin) (Note 3).
Roughly chop the orange and remove any seeds. Place it in a food processor and blitz until it's pureed. Don't worry if there are little bits of rind in the puree.
Add the rest of the cake ingredients to the food processor. Process until the mixture is combined and quite thick.
Scoop the cake batter into a your cake tin. Smooth the top with a spoon or spatula.
Bake the cake for 35-40 minutes (Note 4). The cake is ready when a skewer can be inserted into the middle and there is no wet batter on it (some crumbs are fine). Allow the cake to completely cool in the cake tin.
To serve, dust the cake with icing sugar (Note 5). Store leftovers in an airtight container at room temperature for 1 day, in the fridge for 3 days or in the freezer for up to 1 month.