This cake is really nice as an alternative to Christmas cake.
Make sure you use a thin skinned orange and avoid thick sinned Jaffa oranges as their skin can be bitter.
Use just under half the orange filling to sandwich the cakes together, and spread the rest on top.
Preheat the oven to 180oC/Fan 160oC/Gas 4. Grease and base-line two deep 20cm (8 in) tins with greased greaseproof paper.
1. Place the whole orange in a small saucepan, cover with boiling water and simmer until soft, about 20 minutes. Set aside to cool.
2. When the orange is soft and cold, cut in half and remove any pips. Process the whole orange, including the skin, until medium chunky. Reserve 2 level tablespoons of the orange pulp for the icing, and put the rest back in the processor. Add the remaining cake ingredients and blend until smooth. Avoid overmixing. Divide the mixture evenly between the two tins.
3. Bake in the preheated oven for 25–30 minutes.
4. Leave to cool in the tins for a few moments, then turn out, peel off the paper and finish cooling on a wire rack. Cut each sponge in half to give four separate layers
5. To make the orange filling, cream the soft butter, then add the sifted icing sugar and reserved orange pulp. Sandwich the cakes together with the icing, and use the remaining mixture on top.