Step 2

Put the orange in a small saucepan (for which you have a lid) and cover with cold water. Place the pan over medium-high heat and, when it comes to a boil, reduce the heat to low and cover with the lid. Simmer gently for about 1 hour until the orange is completely soft (use a small knife with a sharp point to check; it should penetrate the orange without too much resistance). With a slotted spoon, lift out the orange (discard the water) and place on a plate. When cool enough to handle, cut the orange into roughly 8 pieces and place in a food processor. Process to a fine puree, then scrape into a small bowl. (*This step can be completed up to 3 days ahead; store the puree in the fridge until it's needed.)

Step 4

Preheat the oven to 175C fan-forced (185C conventional) and line the base and sides of an 18cm round cake tin with baking paper.

Step 6

Sift the flour, baking powder, baking soda and salt into a medium bowl.

Step 8

Combine the butter and sugar in the bowl of a cake mixer and beat with the paddle attachment on medium speed for about 3-5 minutes (the mixture won't be entirely creamy since there's proportionally more sugar than butter). Add the eggs, one at a time, beating well between each addition. Add the orange puree and mix on low speed for a few seconds to combine, then add one third of the flour mix, followed by half of the milk. Maintaining a low speed, mix for a few seconds, then repeat with another third of the flour and the remaining milk. Remove the bowl from the mixer and fold through the remaining third of the flour with a spatula.

Step 10

Scrape the batter into the prepared cake tin and place in the oven. Bake for about 50 minutes, or until a skewer inserted into the middle of the cake comes out clean. Remove from oven and place on a wire rack to cool for 15 minutes, then turn the cake out to cool completely.

Step 12

For the icing, combine the butter, icing sugar and the orange zest in the bowl of the cake mixer with the paddle attachment in place. Beat on medium speed for about 3 minutes, then on low speed, adding the milk and vanilla. Increase speed to high and beat for another 3-4 minutes until light and fluffy.

Step 14

Transfer the cooled cake to a serving plate and spread the icing thickly on top.

Step 15

* Note: Boiling the whole orange tempers the bitterness without diluting the flavour; it takes about an hour to soften, but does so largely unattended, and can be prepared up to three days in advance.