1.Preheat oven to 180°C (160°C fan-forced). Grease a 22cm round loose-bottom cake tin and line the base with baking paper.
2.Place whole orange in a small saucepan. Cover orange with water and weigh down with a small saucer (to keep orange submerged). Bring to boil and simmer for 12-15 minutes until orange is very tender when tested with a skewer. Remove orange with a slotted spoon and cool for 10 minutes. Chop coarsely, reserving juice and discarding seeds.
3.Place orange and any juice in a food processor and pulse briefly until finely chopped. Add butter, sugar, flour, polenta, almond meal, baking powder, egg yolks and liqueur. Pulse until just combined.
4.In a separate clean bowl, whisk egg whites until soft peaks form. In 2 batches, fold egg-whites into orange mixture and pour into prepared cake tin.
5.Bake 40-45 minutes until cooked when tested with a skewer.
6.In a small saucepan, combine orange syrup ingredients over a medium heat. Stir until sugar dissolves. Bring to boil and cook 2 minutes until syrup thickens slightly. Set aside until cake is cooked.
7.Remove cake from oven and poke holes all over the surface with the skewer. Pour orange syrup over the surface of the cake and let stand for 20 minutes so syrup is absorbed into the cake.