Trim the fat from the chicken thighs and cube them into 1-inch pieces.

In a large bowl, mix the dahi, lemon juice, ginger garlic paste, green chilis (if using), garam masala, turmeric, ground cumin, Kashmiri chili powder, and salt. Add the chicken and toss to coat. Cover and marinate for at least 30 minutes, or up to 24 hours.

Crush the green cardamom pods and discard the outer shells.

Melt 2 tablespoons of butter in a pot over medium heat. Add star anise, black cardamom, crushed cardamom seeds, cinnamon stick, and cumin seeds. Sauté for about a minute.

Add in the chopped onions and ginger garlic paste, cooking until the onions brown.

Stir in garam masala, coriander, cloves, and Kashmiri chili powder. Cook for 1 minute.

Mix in the tomato paste, crushed tomatoes, cashews (if using), sugar, and salt. Simmer for 5-10 minutes.

In a separate skillet, cook the marinated chicken on high heat, about 3-4 minutes on each side until browned and cooked through. Set aside.

Discard the whole spices from the sauce and transfer it into a blender. Add remaining butter and blend until smooth, adjusting with water if too thick.

Return the sauce to the pan, add the chicken, and simmer for 5 minutes.

Pour in the heavy cream while stirring and simmer for 2-3 minutes. Add kasuri methi and garnish before serving.