Put whole orange(s) in pan, cover with cold water, bring to boil and cook for 2 hours or until soft. Drain, and when cool, cut oranges in half and remove any big seeds, then pulp everything - pith, peel and all - in food processor.
Preheat oven to 350°. Butter and line 8 inch springform tin.
Add eggs, baking powder, baking soda, almonds, sugar and cocoa to orange in 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 cake tin and bake for about an hour, cake tester should come out pretty well clean. Check after 45 minutes cover with foil to prevent excessive browning, it may need a little less than an hour; all depends on your oven.
Leave cake to cool in tin, on a cooling rack. When cake is cold remove from tin. Decorate with strips of orange peel or coarsely grated zest if you wish, but it is darkly beautiful in its plain, unadorned state.