Place the halved plums into a saucepan with the sugar, cardamom and two tablespoons of the orange juice. Put the pan over a fairly low heat and stir until the sugar dissolves and the plum juices begin to run. Then allow the mixture to bubble gently until the plums are really soft. Remove the pan from the heat.
In a separate pan, bring the remaining orange juice to the boil, then remove the pan from the heat, and sprinkle in the gelatine or agar-agar flakes, following the packet instructions to ensure that it dissolves thoroughly. Add the mixture to the plums. Pour the plum mixture into a processor and whizz to a fine purée. Turn the mixture into a bowl and leave to cool.
In a clean bowl, whisk the egg whites with the salt until they hold soft peaks. Fold them gently but thoroughly into the cooled plum mixture.
Spoon the mixture into a