Whilst the chicken is grilling, prepare the curry. Heat 3 tablespoons butter plus oil in a heavy bottomed pan/ kadai, add whole spices.
When the spices become fragrant add ginger garlic green chilli paste. Sauté until fragrant.
Add tomato purée, mix well with the seasoned butter, cover and let it cook on medium heat until it leaves fat. Stir in between to prevent any sticking at the bottom of the pan. This will take about 3-5 minutes.
Now add salt to season, sugar to balance out the tartness of the tomatoes, kasuri methi, red chilli powder, roasted cumin powder and garam masala. Sauté it on medium heat for another minute.
Add cashew nut paste, sauté it on medium heat another 1-2 minutes until it leaves fat at the sides of the wok.
Add 3/4 to 1 cup water (adjust as per desired consistency, add about 1/2 cup water for very thick sauce), mix well and bring to a boil. Check for seasonings and adjust now.
Add grilled chicken pieces, mix well and let it simmer covered for another 10 minutes.
Finally, add the cream and 1 to 2 tablespoons of butter, mix well and simmer for another 1-2 minutes. Finish it off with a little sprinkle of kasuri methi. Switch off the heat and serve with pilaf or any flatbread!