Wash prawns thoroughly. Heat 5 cups of water in a medium saucepan, and add prawns. Bring to a boil and lower heat. Simmer uncovered for 20 minutes. Remove from flame. Strain liquid and reserve. Shell and devein prawns. Chop onion finely. Peel and chop carrot roughly. Crush garlic and chop deveined prawns finely.
In a large saucepan, heat butter. Add onion, carrot and garlic and fry on medium flame for 3 minutes. Add prawn stock, basil, chicken cube, salt, and tomato puree. Bring to a boil, lower heat and simmer uncoverd for 5 minutes. Remove from flame and blend in batches in a mixie until smooth. Return to flame. In a bowl, mix flour and 1/3 cup water into a smooth paste. Add to soup, stirring constantly. Bring to a boil and allow soup to thicken. Lower heat and stir in cream and half the prawns. Heat gently for 5 minutes without allowing the soup to boil. Serve hot garnished with remaining prawns.