Cashew paste (SKIP IF you are allergic to cashews/ don't like the taste, please use plain yoghurt as substitute) Place the cashews and add 1 cup of water into your blender or food processor and blend on high until completely smooth. Add more water IF needed. It needs to be a thick paste and not runny! Pour into a bowl and set it aside. Don't clean your blender yet - you'll need it again.X

Tomato puree (SKIP if you have store bought) Place roughly chopped tomatoes and blitz. No need to add extra water. Consistency needs to be thick and similar to paste. Set aside.

PART 2: MARINATION

Apply a mixture of red chilli powder, lemon juice and salt to the chicken pieces and set aside for half an hour in the refrigerator.

Hang the yogurt in a muslin cloth for fifteen to twenty minutes to remove extra water. (Add curd to a cloth, tie it up, suspend it from a tap, let the water drip from the cloth)

Marination paste: After this, add the ginger and garlic pastes, red chilli and garam masala powders, salt and mustard oil to the yogurt. Mix well.

Apply this marinade to the chicken pieces and place them in the refrigerator for 2–4 hours.

PART 3 a & b can be done simultaneously to save time! 
Both take roughly the same time; and the chicken is to be added to the gravy when it's almost cooked.

PART 3 a: COOKING THE CHICKEN (10–15 minutes)

Heat skillet, add 2–4 tbsp of butter or oil. Keep on medium - high heat flame 
Add the chicken and brown on both sides, about 10–12 minutes total. You don't need to cook the chicken all the way. Chop the chicken into bite-sized pieces after

PART 3 b: GRAVY (10–12 minutes)

While the chicken is browning, start cooking the curry!

Heat the butter/ oil in a non-stick pan on low flame for a 1 or 2 minutes. Butter heats/ burns real quick; so keep it on low flame. 

Add the green cardamoms, cloves, peppercorns and cinnamon.

Sauté for two minutes, add the ginger and garlic pastes and sauté for two minutes. Add the tomato puree, red chilli powder, salt and half cup of water. Increase flame to high, bring to a boil for 1–2 minutes. 

Bring the mixture to a boil. Reduce the heat and simmer for ten minutes. 

Add the cashew cream, the chopped chicken and stir. Add couple of tablespoons - 1/2 cup of water if it's too thick/ dry and stir on medium flame.

Add the white/ brown sugar or honey and powdered kasoori methi. 

Add the cooked tandoori chicken pieces. Simmer for five minutes. 
Add the fresh cream and stir gently.

Serve hot with naan or parantha.

Notes:

Cashew pieces are less expensive than whole cashews and work well in this recipe. If you don't have a high-powered blender, I recommend soaking the cashews in water for a few hours. Rinse the cashews and discard the water before you blend them.

Place the marinated chicken in the fridge for 2–4 hours for best results. Recommend one hour minimum. 

Be generous with mustard oil in marination process as adds to flavour and makes the chicken softer.

Onions in gravy : I skipped this, but if you like, chop them up finely, add them to the pan after heating the butter and cook them for 4–5 minutes. Once the colour and fragrance have drained, add the spices, pastes, etc.