In a medium bowl, combine the chicken pieces with all the marinade ingredients: yogurt, 1 tbsp ginger-garlic paste, lemon juice, 1 tsp garam masala, 1 tsp Kashmiri chili powder, turmeric, and 1 tsp salt. Mix until the chicken is fully coated. Cover and let it marinate for at least 30 minutes in the refrigerator, or for best results, up to 8 hours.

Heat 1 tbsp of butter and 1 tbsp of oil in a large skillet or Dutch oven over medium-high heat. Once hot, add the marinated chicken in a single layer (cook in batches if necessary to avoid overcrowding). Cook for 3-4 minutes per side, until golden brown and cooked through. Remove the chicken from the skillet and set aside.

In the same skillet, melt the remaining 3 tbsp of butter over medium heat. Add the finely chopped onion and sauté for 5-7 minutes, until soft and translucent.

Add 1 tbsp of ginger-garlic paste and cook for another minute until fragrant. Stir in the ground spices: 1 tsp Kashmiri red chili powder, 1 tsp garam masala, and 1 tsp ground coriander. Cook for 30 seconds, stirring constantly.

Pour in the crushed tomatoes and add the sugar. Bring the sauce to a gentle simmer. While it simmers, drain the soaked cashews and blend them with 1/4 cup of water until you have a completely smooth, creamy paste.

Stir the cashew paste into the tomato sauce. Season with salt to taste. Reduce the heat to medium-low, cover, and let it simmer for 10-15 minutes, until the sauce has thickened and deepened in color. Stir occasionally to prevent it from sticking.

For a silky-smooth sauce, use an immersion blender to blend the sauce directly in the pan until velvety. Alternatively, carefully transfer the hot sauce to a regular blender, blend until smooth, and return it to the skillet.

Return the cooked chicken and any accumulated juices to the skillet. Stir to coat everything in the sauce.

Crush the kasuri methi (dried fenugreek leaves) between your palms to release its aroma and stir it into the curry. Simmer for 2 minutes.

Reduce the heat to low and stir in the heavy cream. Gently heat through for 1-2 minutes, but do not let it come to a full boil. Garnish with fresh chopped cilantro and serve immediately.