Put pumpkin and thin coconut milk in a saucepan. Add salt and bring to a simmering boil until pumpkin is just cooked.
Mix the tamarind paste with water and strain. Heat oil in wok, add mustard seeds and curry leaves; fry until mustard seeds start to pop and splutter.
Add ground spice (A) and ingredients (B). Add a little thick coconut milk to mix and add the prawns. Transfer the cooked pumpkin to the wok and add the remaining thick coconut milk and tamarind juice. Stir and bring to a boil again. Dish out and serve.