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 the onion finely.
Peel and chop the carrot roughly. Crush garlic and chop the deveined prawns finely.
In a large saucepan, heat butter. Add the onion, carrot and garlic and fry on moderate flame for 3 minutes.
Add prawn stock, basil, chicken or fish cube, salt, black pepper, and tomato puree. Bring to a boil, lower heat, and simmer, uncovered, for 5 minutes.
Remove from flame and blend in batches in a mix until smooth. Return to the flame.
In a bowl, mix flour and ? cup water into a smooth paste. Add to the soup, stirring constantly. Bring to a boil and allow soup to thicken.
Lower heat and mix with cream and half the prawns. Heat gently for 5 minutes without allowing the soup to boil. Serve hot and garnished with remaining prawns.