If using chicken legs, remove skin and separate thigh from drumstick. Score the chicken with a sharp knife. Combine tandoori marinade ingredients (whole milk yogurt, salt, kashmiri chili powder, coriander powder, kasoori methi, garam masala, ginger-garlic paste, mustard oil, half lime) in a bowl, adding more chili if necessary for a deep orange color. Add chicken to marinade, cover, and let marinate for 1-3 hours. Add chicken to a wire rack and broil on high for 10 minutes per side, getting the chicken as close to the broiler as possible (if using induction). Then bake at 350F (175C) until fully cooked (8-10 minutes). Let cool, then chop into smaller pieces. Heat a small amount of neutral oil over medium heat, add onions and saute until softened. Add ginger-garlic paste and cook for 2 more minutes, then add the tomatoes and let simmer until they have fully broken down (10-15 minutes). Blend and strain the gravy. Finish the gravy over low-medium heat with cumin, sugar, green chili, garam masala, salt to taste, and kashmiri chili. Stir until combined. Add the chicken, stir until fully incorporated and warmed through, then add butter and let it melt before finishing with heavy cream. Stir to combine.
