Janelle Bloom and can be found on her amazing web site, along with a myriad of other quick, fabulous recipes.
220g chopped navel oranges (this was 1 and a half oranges for me)
185g unsalted butter, softened
1 1/2 cups caster sugar
1 tsp vanilla extract
2 1/4 cups S R Flour
Pulse the chopped orange 3 - 4 times in the bowl of a food processor. Add butter, sugar and extract before pulsing until combined. At this stage, the mixture may look curdled but is fine. Pop the eggs into the bowl and pulse before adding flour. It is important just to pulse until flour is incorporated and not over mix. Pour the batter into a prepared 23cm springform tin and cook in a 170 degree oven (lower for fan-forced) for 60 - 70 minutes or until a skewer comes out clean. Allow to cool for at least 10 minutes before turning out onto a wire rack.
Whipped Orange Buttercream
145g unsalted butter, softened
2 cups icing sugar, sifted
3-4 tbsp freshly squeezed orange juice
Beat butter for 5 minutes until colour lightens. Mix in 1 cup of icing sugar well before adding the orange juice. Beat for a minute. Add last cup of icing sugar and beat with an electric mixer on high for 8-10 minutes. Buttercream should be light in colour and texture. You can add more icing sugar or orange juice until it reaches your desired consistency.