In a bowl, combine the chicken pieces, yogurt, ginger-garlic paste, lemon juice, cumin, garam masala, turmeric, chili powder, and salt. Mix well to ensure the chicken is evenly coated. Cover and marinate in the refrigerator for at least 30 minutes, or up to overnight.

Heat 2 tablespoons of butter in a large skillet or pot over medium-high heat. Add the marinated chicken (discard any excess marinade) and cook until browned on all sides and cooked through, about 8-10 minutes. Remove the chicken from the skillet and set aside.

In the same skillet, melt the remaining 2 tablespoons of butter. Add the chopped onion and cook until softened and translucent, about 5 minutes. Add the ginger-garlic paste and cook for another minute until fragrant.

Pour in the tomato sauce and diced tomatoes (with their juice). Stir in the garam masala, paprika, cayenne pepper (if using), and sugar. Bring the sauce to a simmer and cook for 10-15 minutes, stirring occasionally, to allow the flavors to meld together.

For a super smooth and creamy sauce, use an immersion blender to blend the sauce directly in the pot. Alternatively, you can carefully transfer the sauce to a regular blender and blend until smooth. This step is optional.

Return the cooked chicken to the skillet with the sauce. Stir in the heavy cream and crushed kasuri methi. Simmer for another 5-10 minutes, allowing the chicken to absorb the flavors of the sauce. Season with salt and pepper to taste.

Garnish with fresh cilantro (if using) and serve hot with naan bread or rice.