In a bowl, combine yogurt, ginger garlic paste, chili powder, turmeric, and salt. Coat the chicken thoroughly and let it marinate for 20 minutes.
Add tomato puree, butter, heavy cream, garam masala, and kasuri methi to the slow cooker. Stir gently until combined.
Place the marinated chicken into the sauce without pressing it down too much.
Cover and cook on LOW for 4–5 hours until the chicken becomes tender and the sauce thickens.
Taste and adjust if needed. Add extra cream for richness or more chili powder for heat.
Turn off the slow cooker and let the dish rest for 5 minutes before serving.
Serve warm with basmati rice, garlic naan, or soft roti. Garnish with cream or cilantro if desired.