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 seeds, then pulp everything - pith, peel and all - in a food processor.
Preheat the oven to 350°. Butter and line an 8 inch springform tin.
Add the eggs, baking powder, baking soda, almonds, sugar and cocoa to the orange in the food processor. Run the motor until you have a cohesive cake mixture, but slightly knobbly with the flecks of puréed orange.
Pour and scrape into the cake tin and bake for an hour, by which time a cake tester should come out pretty well clean. Check after 45 minutes because you may have to cover with foil to prevent the cake burning before it is cooked through, or indeed it may need a little less than an hour; it all depends on your oven.
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, but it is darkly beautiful in its plain, unadorned state.