Combine all marinade ingredients except chicken in a bowl and mix. Add chicken and turn well to coat. Cover and marinate in the fridge for a minimum of 3 hours, preferably overnight.

Heat 1 tbsp oil in a non-stick pan over high heat until smoking. Add half the chicken, spread it out, and cook for 2 minutes or until charred. Turn and cook the other side until charred. Remove to a bowl and repeat with remaining chicken.

Wipe the skillet and reduce heat to medium-high. Add oil and butter, then onions, ginger, and salt. Cook until the ginger is golden and onions smell sweet, about 5 - 7 minutes.

Add garlic and paprika to the skillet, and cook for 2 minutes. Add curry sauce spices and cook for another 2 minutes, stirring.

Add tomato passata and water, bring to a simmer, cover, and reduce heat to low. Simmer for 15 minutes, stirring occasionally.

Transfer the curry to a bowl and use a stick blender to puree until smooth. Return sauce to skillet.

Add cream, sugar, and butter, and stir to melt. Add chicken, stir, and simmer for a few minutes until chicken is cooked through.

Optional: Sprinkle with a pinch of extra garam masala before serving. Serve over basmati rice, sprinkled with coriander/cilantro if desired.