In a medium bowl, combine the chicken pieces with all marinade ingredients (yogurt, ginger-garlic paste, turmeric, garam masala, Kashmiri red chili powder, and salt). Mix well, cover, and marinate for at least 30 minutes or up to 4 hours in the refrigerator.

Heat 1 tbsp of oil or ghee in a large skillet over medium-high heat. Cook the marinated chicken in batches until golden brown and cooked through. Set aside.

In the same skillet or a Dutch oven, melt the butter/ghee over medium heat. Add the chopped onion and sauté until soft, about 5-7 minutes. Add the ginger-garlic paste and cook for 1 minute more until fragrant.

Stir in the coriander, cumin, and garam masala, and cook for 30 seconds. Add the crushed tomatoes and the soaked, drained cashews. Bring to a simmer, reduce heat, and cook for 15 minutes, stirring occasionally.

Carefully transfer the sauce to a blender or use an immersion blender to blend the sauce until it is completely smooth and creamy. Return the sauce to the pot.

Stir the cooked chicken into the blended sauce. Add the heavy cream, crushed kasuri methi, and optional sugar. Simmer for 5-10 minutes for the flavors to meld. Serve hot.