Heat oil in a large saucepan and cook the onion until lightly browned.
Make the spice mix: combine all ingredients in a food processor or spice grinder and grind to a powder. Add to the onions and cook until fragrant.
Add the prawns, mangoes, curry leaves. Mix the coconut milk powder into the boiling water and add to the curry. Simmer 5 minutes, until prawns are just done. Remove from heat and stir in the cream.