To make prawn juice, take prawn heads and pound in a mortar with a pestle until real fine and juicy. Mix with 125ml (1/2 cup) water and pour through a strainer. Discard pulp and set aside prawn juice. You should have about 115ml juice.
Cut a small cross in the base of each tomato and cover the tomatoes with boiling water in a heatproof bowl. Stand for 30 seconds and remove with a slotted spoon to a bowl of cold water. When the tomatoes are cool enough to handle, peel them with a small, sharp knife and discard the skin. Roughly chop the flesh.
Saute onion and garlic in olive oil until onion is translucent. Add ginger, curry powder, salt and pepper and mix thoroughly.
Melt 40 g of the butter in a medium frying pan. Add the onion and garlic and cook over medium heat until the onion is light golden, about 10 minutes. Add the curry powder, flour and crumbled stock cube and cook, stirring, for 2 minutes. Add the tomato and water. Bring the sauce to the boil, reduce the heat to low, cover the frying pan with a lid and simmer for 8 minutes, or until the sauce thickens slightly.
Add the prawn juice, stirring continuously to make a smooth mixture. Let simmer 5 minutes.
Meanwhile, peel and devein the prawns, leaving the tails intact. Melt the remaining butter in a small frying pan over medium-high heat. Add the prawns and cook until they turn pink. Spoon the prawns and butter mixture into the curry sauce stir for a further minute, or until the prawns are well coated with the sauce. Serve with boiled rice.
Add prawns, lemon juice and green chilli. Mix well and cook 2 minutes.
Add coconut milk and continue mixing. Cover and cook 5 minutes more. Serve hot with rice.