Pre-heat the oven to Gas Mark 4 (180°C). 

Grease and line a 22cm square baking tin with parchment paper. 

Cream together the butter and sugar, until pale and fluffy. Then add the egg yolks and combine. 

Add the juice and zest of the oranges, and the Aperol, and again combine. The acid in the juice will react with with butter, and it'll look odd, but don't worry, it will all come together!

Sift in the flour, and stir until it's all just combined. 

In a separate bowl, beat the egg whites until they are white and fluffy. I cheated a little and set them going in my food processor while I beat together the other ingredients by hand.

Take a spoon of the fluffy egg whites and roughly mix into the cake batter. This little sacrifice will lighten the batter and make folding the rest of the egg whites in a little bit easier. 

Finally, tip in the last of the egg whites, and fold in gently, again until it's all just combined. Pour into the cake tin, smooth the top a little and pop it into the oven. 

The cake will take around 30 minutes to bake, but to test, take a skewer and insert it into the centre of the cake. If it comes out clean, the batter is cooked all the way through. If it's smeared with batter, pop it back in for another five minutes, and test again. 

Remove from the oven and set to cool on a rack. Or if you're impatient like me, put the kettle on while you slice it frantically so you can eat it as soon as possible.