For the butter chicken / Pangasius sauce, return saucepan to medium-high heat and add ghee, onion, garlic, cinnamon, ginger and fresh chilli. Cook, stirring occasionally, for 5 minutes or until onion is softened. Add spices and 1/2 tsp ground black pepper, and cook, stirring, for 2 minutes to infuse. Add coconut, cashew, tomato, sugar, tamarind paste and cream. Reduce to a gentle simmer and cook, stirring occasionally, for 15 minutes to infuse. Discard cinnamon, transfer sauce to a blender and whiz until smooth. Chop chicken thighs into 3-4 pieces and stir through sauce.