In a mixing bowl, combine the boneless skinless chicken thighs with plain yogurt, minced garlic, grated ginger, garam masala, cumin, chili powder, and salt. Cover and refrigerate for at least 30 minutes.

Heat vegetable oil in a large skillet over medium-high heat. Add the marinated chicken and cook for 4-5 minutes until browned. Remove chicken and set aside.

In the same skillet, reduce heat and add unsalted butter. Add chopped onion and sauté for 2-3 minutes until translucent. Stir in garlic, ginger, and remaining spices, cooking for 1 more minute.

Pour in the tomato puree, stir, and let simmer for 3-4 minutes. Gradually add heavy cream, sugar, and water, mixing until smooth.

Return the chicken to the skillet, submerging it in the sauce. Simmer for 7-8 minutes until fully cooked and sauce thickens.

Taste and adjust seasoning, then serve hot with steamed rice or naan.