First make the marinade: in a food processor or blender, briefly whizz the chillies, garlic and cumin seeds then add the remaining marinade ingredients and whizz to combine. Transfer to a bowl, add the chicken pieces and stir well. Marinate in the fridge at least 3 hours.
Combine the garam masala, ground coriander, paprika and chilli in a small bowl and set aside.
Melt 20g of the butter in a frypan and fry the chicken (reserving the marinade) until browned, then set aside.
To the frypan, add the onion, garlic and ginger. Add the cardamom pods, cloves and ground spice mix and fry one minute.
Add reserved marinade, passata and lemon juice. Simmer 10 minutes then add the chicken and simmer until the chicken is cooked through, about 20 minutes.
10 minutes before serving, add the remaining 40g butter and the cream and stir through.