In a nonstick frying pan, dry-roast the cashews and curry powder over a low heat for 2–3 minutes. Transfer to a coffee grinder and blend until smooth.
Add this mixture to a food processor or blender with the garlic, ginger, vinegar, tomato purée and half the yogurt. Process until smooth. Transfer to a mixing bowl with the remaining yogurt and the chicken. Mix well, cover and marinate in the refrigerator for 24 hours.
Melt the butter in a large nonstick pan and add the onion, cinnamon and cardamom pods. Stir-fry over a medium heat for 6–8 minutes or until the onion is soft. Add the chicken mixture and cook, stirring, for 10 minutes. Season to taste.
Stir in the chilli powder, tomatoes and stock, bring to the boil and reduce the heat. Simmer, uncovered, for 40–45 minutes, stirring occasionally. Add the cream and cook gently for 4–5 minutes. Garnish with the chopped coriander and a drizzle of single cream. Serve immediately with boiled rice or Naan (see