Wipe the fruit well and place in a pot with enough cold water to cover. Bring to the boil and simmer briskly with the lid on for 20 minutes, topping up with hot water if the fruit isn’t floating. Remove from the pan and cool for 20 minutes.
Preheat the oven to 170°C/340°F/Gas 3-4. Cut the boiled fruit in half and remove seeds, then place in a food processer, skin and all. Add the remaining ingredients and purée for 30 seconds; wipe the sides of the bowl back into the mix and purée for another 20 seconds. Pour the thick batter into a spring-form cake tin and bake for about 50 minutes. Check after 40 minutes by inserting a thin knife into the cake. It may come out with a tiny bit of batter, but so long as it is cooked batter you are all right – remember, this is a moist cake! Remove from the oven and, when cool, take from the cake tin and leave to sit for a few hours before eating.
This cake is delicious enrobed with chocolate ganache. Gently melt the grated bitter chocolate in a heatproof bowl over a pot of boiling water. Stir in the double cream and spread on the cold cake.