Combine chicken, salt, garlic powder, sweet paprika, curry powder, and Greek yogurt in a medium bowl. Mix thoroughly until every piece is coated evenly. Let marinate for at least 15 minutes while dicing onion and mincing garlic.

Heat vegetable oil in a large skillet over medium-high heat. Add marinated chicken in a single layer and cook for 8 to 10 minutes, stirring occasionally, until golden brown and internal temperature reaches 165°F. Transfer chicken to a plate, leaving oil in pan.

Reduce heat to medium and add 1 tablespoon butter to the skillet. Scrape up browned bits from bottom of pan, adding 1 to 2 tablespoons water if needed. Add minced garlic and diced onion with a pinch of salt. Sauté for 3 to 4 minutes until onion is translucent and fragrant.

Stir in tomato sauce and sugar. Let simmer for 2 to 3 minutes. Return chicken to skillet and mix well. Pour in heavy cream slowly, stirring as sauce transforms to a peachy-orange color.

Add cayenne pepper if using, garam masala, curry powder, and black pepper. Reduce heat to low and simmer for 10 minutes. Sauce should coat the back of a spoon. Taste and adjust salt as needed.

Remove from heat and stir in remaining 2 tablespoons cold butter until completely melted. The cold butter prevents cream separation and creates glossy finish. Garnish with fresh parsley if desired. Serve hot with naan bread and steamed rice.