Prep all ingredients.
In a large skillet over medium heat, melt butter and oil. Add chicken, onion, and garlic. Cook for about 10 minutes, stirring occasionally, until the onion has softened and turned translucent.
Mix in tomato paste, curry paste, curry powder, tandoori masala, and garam masala until smooth. No lumps of tomato paste should remain.
Transfer mixture to a slow cooker. Stir in cardamom pods, coconut milk, and yogurt. Season with salt.
Cook on High for 4 to 6 hours or on Low for 6 to 8 hours until chicken is tender and sauce has thickened.
Remove and discard cardamom pods before serving.
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