Heat 2 tbsp of oil in a large skillet over medium heat. Add chicken and let it cook. It should take 3 to 4 minutes. Once chicken is cooked, take it out in a bowl and keep it aside.
Now in the same skillet add the rest of the oil and butter on a medium heat. Fry onions until they are golden brown(it should take about 5 minutes).
Add tomato sauce, red chili powder, butter chicken masala, salt, kasoori methi, and garam masala. Cover the pan and let it cook for about 7 minutes, stirring occasionally until oil starts to separate.
Now at this point add cashew paste and brown sugar, mix it well. Add milk and heavy cream and let it simmer for 1 minute. Once the sauce is well combined add chicken and cover the pan. Let it cook on a medium to low heat for about 5 minutes stirring occasionally. Once sauce is a little thick and chicken is tender turn off the heat. Garnish with cilantro, roasted nuts, and serve rice and fresh naan.