In a small bowl, combine 4 tsp garam masala, 3 tsp chilli powder, 2 tsp ground coriander, cumin, paprika and turmeric and 1.5 tsp salt. In a medium bowl, combine your chicken with half the spices. Keep other half for later.
Heat up around 1 tbsp veg oil in a pan over high heat. Once it just begins to smoke, space out your chicken (work in 2 batches if needed). Leave to fry until it begins to char, then flip each piece and char on the second side. You're not looking to cook the chicken at this point, this is purely to lock in the spices and caramelize the outside (a little char = extra and authentic flavour).
Remove chicken from pan and place into slow cooker. Lower heat to medium and add 2 tbsp butter. Add onion and fry until it begins to soften, then add ginger and garlic. Continue frying until it's all softened and beginning to catch colour. Stir in 2 tbsp tomato puree, fry for a minute or so, then scrape everything into the slow cooker.
Into the slow cooker pour in passata and water, then add your remaining spices and sugar. Give it a good stir then pop on the lid and cook on HIGH for 3-4hours or LOW for 5-6hours.
Remove lid and flick the cooker to 'warm'. Allow the curry to cool slightly, or at least stop bubbling then stir in the cream (important cream at room temp and curry cools slightly or the cream may curdle).
Serve with coriander/cilantro, rice and naan!