Marinate the chicken pieces with half of the powdered spices, yogurt, ginger garlic paste, salt, butter and lemon overnight or at least for 2 hours.
Pre heat the oven to 220 C and bake the chicken for 30 minutes or till just done. (Remember the chicken pieces will again get cooked in the gravy) (Every oven and the settings are different, so use your best judgement when it comes to the temperature setting and the timing)
Do not discard any liquid from the baked chicken.
Heat oil in a pan for gravy. Add onion and fry till golden brown in color.
Add the green chillies and ginger garlic paste. Fry till the raw smell vanishes from the ginger garlic paste.
Add the red chilli powder, tomato puree, and the remaining half of the powdered spices. Cook for couple of minutes and remove from flame. Blend this in a mixer or hand blender to make a paste.
Transfer again to pan and add the baked chicken. Stir well.
Close the lid and cook on low heat for about 10 minutes. By now the butter starts separating from it.
Add the cream and switch off the flame.
Serve hot with nan or roti.