1.Preheat oven to moderate, 180°C (160°C fan-forced). Grease a deep 20cm-round cake pan; line the base and side with 2 layers of baking paper.
2.Combine caster sugar and the water in a large frying pan. Stir over medium heat, without boiling, until sugar dissolves. Bring to the boil, then simmer, without stirring, uncovered, for about 5 minutes or until syrup thickens slightly. Add orange slices; simmer gently, uncovered, for about 7 minutes or until rind is tender, turning slices halfway through cooking time.
3.Remove from heat; using tongs, lift orange slices from syrup and transfer to cover base of prepared pan, slightly overlapping each slice. Reserve syrup.
4.Add extra water to reserved syrup in pan; bring to boil. Reduce heat; simmer, uncovered, without stirring, for about 5 minutes or until syrup is a light honey colour. Pour two-thirds of the hot syrup over orange slices in pan; reserve remaining syrup.
5.Beat butter, rind and brown sugar in a small bowl with an electric mixer until light and fluffy. Beat in eggs, one at a time, until combined.
6.Transfer mixture to a large bowl; using wooden spoon, stir in combined sifted flours, soda and cocoa, then almond meal, polenta, sour cream, juice and chocolate. Carefully spread mixture over oranges in prepared pan.
7.Bake cake in a moderate oven for about 1 1/4 hours. Stand cake for 15 minutes, then turn onto serving plate.
8.Spoon reserved syrup over oranges.