I came up with this idea one day to use up extra oranges,basing it on a plain sponge cake recipe.The recipe may look long and involved but it's really very easy! The only special equipment you need is a thermometer-and patience:don't peek at the cake too soon;the draft of air will make it fall,and you'll end up with a crater in the middle of your cake.Just keep an eye on it through your oven window and everything will turn out great.