1. Combine sugar and water in a large saucepan; stir over medium heat until sugar is dissolved. Bring to the boil; boil, uncovered, without stirring, for about 10 minutes or until a lovely caramel colour. Keep a close eye on the pan. Add the butter and stir briskly with a metal spoon. Be prepared for the mixture to froth up.
2. Meanwhile, preheat the oven to 180°C (160°C fan-forced). Grease a deep 22cm round cake pan; line base and side with baking paper.
3. Finely grate the rind from the oranges. Peel oranges, removing all the white pith. Cut oranges in half from top to bottom, then cut out the centre in a V-shape to remove the white membranes. Cut into 1cm slices. Arrange the orange slices over the base of pan; pour caramel over oranges.
4. Beat extra butter, rind and extra sugar in a medium bowl with an electric mixer until light and fluffy. Add eggs one at a time, beating well after each addition. Stir in dry ingredients and sour cream in two batches. Pour the batter carefully over the oranges; smooth top.
5. Bake for about 1 hour 10 minutes, or until a skewer inserted into the centre of the cake comes out clean. Stand for 10 minutes, then invert onto a serving plate.
6. Serve warm with double cream or vanilla ice-cream, if desired.

Suitable to freeze. Not suitable to microwave.