Combine chicken chunks with salt, garlic powder, paprika, curry powder, and Greek yogurt in a large bowl. Toss to coat, cover, and refrigerate for at least 15 minutes and up to overnight.

Heat half the butter with the vegetable oil in a large skillet over medium-high heat. Add marinated chicken in a single layer and sear for 4 to 5 minutes total, turning once, until lightly golden. The chicken does not need to be fully cooked through. Transfer to a plate.

In the same pan, melt the remaining butter over medium heat. Add minced garlic and diced onion. Cook for 4 to 5 minutes, stirring occasionally, until the onion is soft and translucent.

Stir in tomato sauce, sugar, salt, black pepper, curry powder, and cayenne if using. Bring to a gentle simmer and cook for 2 minutes.

Pour in the heavy cream slowly while stirring continuously. Reduce heat to medium-low and simmer for 5 to 7 minutes until the sauce is deep orange and coats the back of a spoon. Do not boil as this can cause the cream to separate.

Return seared chicken and resting juices to the pan. Sprinkle in the garam masala and stir gently. Simmer for 8 to 10 minutes until chicken is cooked through. Taste and adjust salt or sugar as needed.

Garnish with chopped parsley and serve hot with warm naan and steamed basmati rice.