The truth is that my mother in law shared this recipie with me, it's very tasty!
Heat butter in a wide heavy pot over moderately low heat until foam subsides, then cook onions, garlic, and ginger, stirring until softened, about five minutes. Add curry powder, salt, cumin and cayenne and cook, stirring 2 minutes.
Add chicken and cook, stirring to coat, 3 minutes.
Add tomatoes, including juice, and cilantro and bring to a simmer, then cover and simmer gently, stirring occasionally, until chicken is cooked through, about 40 minutes.
Just before serving: Pulse cashews in a food processor or chop them very finely as this will make the sauce thick, then add to curry along with the yogurt or crème fraiche and simmer gently, uncovered, stirring, until sauce is thickened, about five minutes.
Accompaniment: cooked basmati rice, jasmine rice or cuscus.