For the marinade: Briefly blend the chillies, garlic and cumin seeds, then add the remaining ingredients and blend. Transfer to a bowl and add chicken pieces, coating evenly in the marinade. Chill in fridge for at least 3 hours.
Mix together all the ground spices, and set aside.
Fry the chicken (save any leftover marinade) in 20g of the butter until browned, then remove from pan and set aside.
To the same pan, add the onion, garlic and ginger. Add the cardamoms, cloves and ground spices, gently frying for a minute. Add leftover marinade, passata and lemon juice, simmer for 10 minutes then add the chicken into the sauce and leave simmering until chicken is cooked through.
Ten minutes before you dish up, add the remaining 40g butter and the cream and stir through.