Soak the beans in a bowl of cold water overnight and then drain.
Pat the prawns dry between several layers of kitchen paper. Dust lightly with the chilli powder and a little salt.
Heat the oil in a large saucepan and fry the prawns briefly on both sides until they have turned pink. Stir in the tomato paste, sugar and 2 tablespoons water. Cook over a gentle heat, stirring, for 1 minute, then scrape out on to a plate.
Add the bacon to the pan and cook until browned. Stir in the drained beans, onions, bay leaves and 1 litre (1¾ pints) cold water and bring to a gentle simmer. Reduce the heat, cover and cook very gently for 40 minutes or until the beans are soft.
Remove the bay leaves. Blend the soup using an immersion blender or in a food processor. Return to the pan and reheat gently, stir in half the prawns and season to taste with salt and pepper.
Ladle the soup into serving bowls and spoon the remaining prawns and their cooking juices on top. Serve garnished with roughly chopped parsley.