In a seal-able fridge container, mix the buttermilk, yogurt, curry powder, coriander leaves and lime juice. Add the cubed chicken. Make sure all the pieces are covered and put in the fridge for 3 hours.
When you are ready, heat the butter in a saucepan.
Fry the onions and garlic. Add the buttermilk chicken. Bring to a boil. Add the black pepper and salt.