Put the whole orange or oranges in a pan with some cold water, bring to the boil and cook for 2 hours or until soft.
Drain and, when cool, cut the oranges in half and remove any big pips.
Pulp everything – pith, peel and all – in a food processor.
Preheat the oven to gas mark 4/180°C.
Butter and line a 20cm springform tin.
Add the eggs, baking powder, bicarbonate of soda, almonds, sugar and cocoa to the orange in the food processor.
Run the motor until you have a cohesive cake mixture, but still slightly knobbly with the flecks of puréed orange.
Pour and scrape into the cake tin and bake for an hour, checking after 45 minutes.
Leave the cake to get cool in the tin, on a cooling rack.
When the cake is cold you can take it out of the tin.
Decorate with strips of orange peel or coarsely grated zest if you so wish.
