In a large mixing bowl, combine the chicken, salt, garlic powder, sweet paprika, curry powder, and Greek yogurt. Mix well and let it sit for at least 30 minutes.

Heat 2 tablespoons of vegetable oil in a non-stick skillet over medium-high heat. Add the marinated chicken and cook until browned and almost cooked through (about 8-10 minutes). Remove from the skillet and set aside.

In the same skillet, add the remaining tablespoon of oil and 1 tablespoon of butter. Add the diced onion and cook until soft and translucent (approximately 5 minutes). Stir in the minced garlic and cook for another minute.

Pour in the tomato sauce and stir well. Add sugar, salt, black pepper, heavy cream, cayenne pepper (if using), garam masala, and the additional curry powder. Bring to a simmer and cook for about 10 minutes.

Return the chicken to the skillet. Stir well to combine, and let it simmer for another 10 minutes.

Add the remaining 2 tablespoons of butter to the sauce. Stir until melted and well combined.

Garnish with chopped parsley if desired and serve hot with naan bread or steamed rice.