Whisk Greek yogurt, 1 tablespoon garam masala, lemon juice, cumin, cinnamon, cayenne pepper, black pepper, ginger, and salt together in a bowl and pour into a resealable plastic bag. Add chicken, and work the marinade into the chicken until it is coated. Squeeze out excess air and seal the bag. Marinate in the refrigerator for 1 hour to overnight.

Transfer chicken and marinade to a slow cooker. Add heavy cream, tomato sauce, garlic, jalapeno pepper, remaining 1 tablespoon garam masala, butter, cumin, paprika, and salt; mix with a ladle to combine.

Cover and cook on Low for 5 hours, or on High for 3 hours, until chicken juices run clear.

Whisk cornstarch and cold water in a small bowl until completely combined, adding more water if necessary. Add to the slow cooker. Mix in ketchup. Cook on High for an additional 20 minutes. Serve with cilantro.