First, prepare and clean the prawns. Remove the head and cut a slit in the back with a sharp knife. Pull out the entire vein and rinse under cold water.
Heat 2 tablespoons of ghee or oil in a saucepan. Add the mustard seeds and fry for a few minutes until they pop. Add the onion, ginger and garlic and cook for a few minutes until fragrant. Stir in the garam masala, cumin, coriander and chilli powder – do not let the spices burn!
Add a little more ghee and cook for a few minutes. Now add the sugar, salt, pepper, cardamom, tomatoes, tomato paste, chicken stock and curry leaves. Bring to the boil and cook until the sauce becomes fragrant. Pour the sauce into a food processor and blend until smooth. Return to the saucepan and add the cream and prawns. Bring the sauce to a simmer, adjust the seasoning and cook simmering until the prawns change colour; remove from the heat. Stir in the chopped butter a few pieces at a time.
Spoon the curry into a serving bowl and garnish with chopped coriander. Serve with lots of steaming hot basmati rice.