Bring a large pot of water to the boil. Wash the oranges and cook in the boiling water for 2 hours. Drain, allow to cool to room temperature, then puree. This step can be done ahead of time. Preheat the oven to 160°C. Grease and line a 22 cm spring form cake tin with baking paper. Beat the eggs and caster until well combined. Stir in the orange puree followed by the almond meal and baking powder. Pour the batter into the prepared tin and dust the top with extra caster sugar. Bake for 1-1¼ hours, until the top is golden and a skewer inserted into the centre comes out clean. Allow to cool in the tin. Dust with icing sugar, cut into slices and serve.