In a large bowl, add the chicken, yoghurt, lemon juice, cumin, chilli powder and paprika and mix well. Put in the fridge for a couple of hours, or it can be kept in there overnight.
In a large pan, heat some oil and then add the onion, garlic and ginger and cook on low for 4-5 minutes until softened.
Add the garam masala and tomato paste and stir.
Add the chicken mixture and the coconut milk and stir well.
Transfer it to a slow cooker.
Cook on high for 3 hours or on low for 6 hours. Add the butter 10 minutes before finished and stir.
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Notes
You can use light coconut milk instead of full fat in this curry, but you won't get the same creamy flavour.
Add a little crunch to this curry by serving with some cashew nuts on top.
This is a really mild curry, making it perfect for serving up to the whole family. However you you can always add a finely diced red chilli to the sauce, or just sprinkle some red chilli flakes.
You can give this an extra protein boost by adding some quick cook lentils at the same time as the coconut milk.
An easy way to add a little extra flavour, texture and creaminess is by adding a spoonful of peanut butter (or almond or cashew).
Add some fresh coriander (cilantro) before serving to lighten it up a little.