For the marinade, mix all of the marinade ingredients together in a bowl until well combined. Add the chicken breasts and make sure they’re well covered in the marinade. Set aside to marinate in the fridge for 12 hours, or overnight.
When the chicken has marinated, preheat the grill to its highest setting. Remove the chicken breasts from the marinade, shaking off any excess, and grill for 8-9 minutes on each side, or until the outside is golden-brown and the inside is cooked through. Baste the chicken breasts with the butter and a little lemon juice. Set aside.
Meanwhile, for the sauce, heat the butter in a frying pan over a medium heat and fry the onion for 4-5 minutes, or until softened. Add the ginger and garlic and cook for a further minute, then stir in the tomato purée, chillies and chilli powder and cook for another 2-3 minutes.
Reduce the heat, stir in the cream and simmer for a further minute, then add the chicken breasts to the sauce. Season, to taste, with salt and freshly ground black pepper.
To serve, place the butter chicken breasts onto serving plates and drizzle over the sauce. Sprinkle over the coriander.