Combine coriander seeds, black peppercorns and turmeric powder. Grind to a paste with a little water and set aside.
Heat oil and saute the mustard seeds until they crackle. Add the onion, garlic, ginger and curry leaves, and saute until the onions become translucent. Add the chicken and saute until golden brown.
Add the curry paste mix, and saute for another minute, stirring constantly to avoid sticking. Add the vinegar and 1/2 cup of water and simmer for 2 minutes. Add the coconut milk and mango cubes and salt to taste.
Gently simmer for a further 5 minutes, taking car not to allow the coconut milk to boil.