Preheat the oven to 180°C /160° fan/Gas 4. Lightly grease a 24cm square cake tin and line with baking parchment.
Place the orange in a pan and cover with cold water, bring to the boil then simmer gently for 30 minutes until the orange is soft. Place the orange in a bowl of cold water and leave to cool.
Roughly chop the whole orange and discard any pips then place the orange in a food processer and blend until smooth. A stick blender also works well.
Sieve the flour, ground almonds, baking powder and cinnamon into a bowl. In a separate bowl whisk together the sugar and eggs until thick and creamy. When it is ready the whisk should leave a trail on top of the mixture.
Using a spatula gently fold the flour mixture and pureed orange into the sugar and eggs.
Pour the mixture into the prepared tin then bake in the preheated oven for 30 minutes, until the sponge is golden and firm to touch. Allow to cool slightly in the tin before turning out onto a wire rack to cool completely.