Gather all ingredients.
Heat butter and vegetable oil in a large skillet over medium heat. Stir in chicken, onion, and garlic. Cook and stir until onion has softened and turned translucent, about 10 minutes.
Stir in tomato paste, curry paste, curry powder, tandoori masala, and garam masala until no lumps of tomato paste remain.
Pour mixture into a slow cooker; stir in cardamom pods, coconut milk, and yogurt. Season with salt.
Cook on High for 4 to 6 hours or Low for 6 to 8 hours, or until chicken is tender and sauce has reduced to the desired consistency. Remove and discard cardamom pods before serving.
