01 - Combine chicken with yogurt, lemon juice, turmeric, cumin, garam masala, chili powder, and salt in a large bowl. Mix thoroughly to coat all pieces. Cover and let marinate for at least 10 minutes, or up to 1 hour for deeper flavor penetration.
02 - Heat 1 tablespoon butter and vegetable oil in a large skillet over medium-high heat. Add marinated chicken pieces and cook for 5 to 6 minutes, turning occasionally, until lightly browned on all sides but not fully cooked through. Remove chicken from skillet and set aside, reserving any juices.
03 - Add remaining 1 tablespoon butter to the same skillet. Sauté chopped onions for 3 to 4 minutes until soft and golden brown. Add minced garlic and grated ginger; cook for 1 minute until fragrant, stirring constantly to prevent burning.
04 - Stir in ground coriander, paprika, and cayenne pepper. Cook for 30 seconds to bloom the spices. Add crushed tomatoes and sugar; simmer for 5 minutes, stirring occasionally, until the mixture begins to thicken slightly.
05 - Return chicken and any accumulated juices to the skillet. Simmer for 8 to 10 minutes, or until chicken is cooked through and sauce has thickened to a creamy consistency. Stir occasionally to ensure even cooking.
06 - Reduce heat to low. Stir in heavy cream and simmer for 2 additional minutes, allowing flavors to meld. Taste and adjust salt as needed. The sauce should be rich and velvety.
07 - Transfer to a serving dish and garnish with chopped fresh cilantro. Serve immediately while hot, accompanied by steamed basmati rice or warm naan bread for soaking up the sauce.