Place 3 oranges in a saucepan and cover with cold water. Bring to the boil, then reduce heat to low and simmer for 2 hours, topping up water if necessary. Drain, discarding liquid. Cool slightly, then roughly chop, discarding seeds.
Preheat the oven to 170°C. Grease and line the base and side of a 28cm loose-bottomed cake pan. Transfer the chopped orange to a food processor and whiz until smooth. Measure out 450g of the puree, discarding any remainder.
Beat eggs and 1 1/3 cups (300g) sugar until thick and pale. Add the almond meal, baking powder, spices, cocoa and orange puree, then stir to combine. Spoon into the pan and bake for 1 hour, then loosely cover with foil and cook for a further 20 minutes or until a skewer inserted into the centre comes out clean.
Cool in the pan for 10 minutes, then turn out onto a wire rack to cool completely.
Meanwhile, for syrup, finely pare the zest of remaining orange, then juice. Place juice in a bowl. Combine remaining 1 cup (220g) sugar and 1/4 cup (60ml) water in a pan over medium heat. Bring to a simmer and cook, without stirring, for 8 minutes or until a dark caramel forms. Carefully add orange juice (mixture may spatter) and stir to combine. Remove from heat and stir in zest. Cool completely.
Dust cake with extra cocoa and serve topped with ice cream, drizzled with orange syrup.