In a large bowl, combine skinless boneless chicken thighs with salt, garlic powder, sweet paprika, curry powder, and Greek yogurt. Let the chicken marinate for at least 15 minutes.

Heat a splash of vegetable oil in a large skillet over medium-high heat. Add the marinated chicken and cook for about 8-10 minutes until golden brown and fully cooked. Remove from skillet.

Reduce heat to medium and add the remaining butter to the skillet. Let it melt, then add minced garlic and diced onion. Sauté for 3-4 minutes until the onion is translucent.

Stir in tomato sauce and sugar, and let it simmer for 2-3 minutes. Return cooked chicken to the skillet and add heavy cream, stirring gently.

Add cayenne pepper, garam masala, additional curry powder, and black pepper. Let simmer for about 10 minutes.

Stir in remaining cold butter until melted. Remove from heat and garnish with freshly chopped parsley.

Serve the homemade butter chicken hot with warm naan and fluffy steamed rice.