01 - In a large mixing bowl, combine the yogurt, lemon juice, ground cumin, garam masala, turmeric, salt, and coriander. Add the chicken pieces and toss thoroughly to coat evenly. Let marinate for at least 15 minutes at room temperature, or cover and refrigerate overnight for deeper flavor penetration.
02 - Melt 1 tbsp of butter in a skillet over medium heat. Add the chopped onion and sauté until golden and softened, about 4 minutes. Stir in the minced garlic and ginger, cooking for an additional minute until fragrant. Transfer the entire mixture to the crockpot.
03 - Add the marinated chicken along with all of the marinade to the crockpot. Pour in the crushed tomatoes, paprika, cayenne pepper, sugar, and the remaining 1 tbsp of butter. Stir everything together until well combined.
04 - Cover the crockpot and cook on low for 6 hours, or on high for 3 to 4 hours, until the chicken is fall-apart tender and the flavors have melded.
05 - Stir the heavy cream into the sauce. Taste and adjust salt and spice levels as needed. Cook uncovered for an additional 10 minutes to allow the sauce to thicken slightly.
06 - Ladle the butter chicken into bowls and garnish generously with chopped fresh cilantro. Serve alongside steamed basmati rice or warm naan bread.