Heat the ghee in a saucepan over a medium-high heat. Add the marinated chicken and cook for 3–4 minutes, turning occasionally, until lightly browned. The chicken does not need to be completely cooked at this stage. Transfer it to a bowl and set aside.
Reduce the heat to medium. Add 10g of the butter to the same pan, followed by the cumin seeds and cinnamon stick. Cook for approximately 30 seconds, until fragrant. Add the onion and cook gently for 6–8 minutes, until soft and lightly golden.
Add the ginger and garlic and cook for 1 minute. Stir in the ground cumin, ground coriander, chilli powder and tomato purée. Cook for a further 30 seconds.
Add the chopped tomatoes, ground cashew nuts, hot water and a little salt. Simmer gently for 10–12 minutes, stirring occasionally, until the tomatoes have softened and the sauce has thickened slightly.
Remove the cinnamon stick. Blend the mixture with a hand blender until smooth.
Pour in 100 ml of hot water and cook for about 10 minutes, or until most of the water has evaporated.
Blend the mixture into a smooth sauce using a hand blender
Return the chicken and any resting juices to the sauce. Simmer gently for 8–10 minutes, or until the chicken is completely cooked through. Add a splash of hot water if the sauce becomes too thick.