01 - In a large bowl, combine the Greek yogurt, lemon juice, garam masala, ground cumin, ground coriander, chili powder, turmeric, and salt. Add the chicken pieces and toss to coat evenly. Cover and refrigerate for at least 30 minutes, or overnight for deeper flavor penetration.
02 - Melt 2 tablespoons of unsalted butter in a skillet over medium heat. Add the finely chopped onion, minced garlic, and grated ginger. Sauté until the onion turns soft and golden, approximately 5 minutes.
03 - Transfer the sautéed aromatics to the crockpot. Add the marinated chicken along with all the marinade, tomato sauce, garam masala, smoked paprika, ground cumin, cinnamon, and cayenne pepper if using. Stir thoroughly to combine all ingredients.
04 - Cover the crockpot and cook on LOW for 5 to 6 hours, or on HIGH for 2.5 to 3 hours, until the chicken is fork-tender and cooked through.
05 - Approximately 20 minutes before serving, stir in the heavy cream and remaining 2 tablespoons of butter. Season with salt and freshly ground black pepper to taste, then replace the lid and allow the sauce to finish melding.
06 - Garnish generously with fresh cilantro. Serve hot alongside steamed basmati rice or warm naan bread.