Wash the oranges. Place in a pan of water with the skins left on.
Bring to a boil. Reduce to simmer and boil for 2 hours.
Remove from the water and set aside to cool.
Place in a blender and blend to a puree.
If you have a pressure cooker you can reduce the cooking time to 45 minutes.
Preheat the oven to 160°C. ( conventional - not fan forced)
Grease and line a 22 cm springform cake tin with baking paper.
Beat the sugar and eggs together using a hand blender.
Stir in the orange puree, followed by the ground almonds and baking powder.
Pour the mixture into the tin and place it in the centre of the oven to bake for between an hour and an hour and fifteen minutes.
Remove from the oven, set aside to cool
Decorate with a little orange zest.