Soak cashew in milk for 30 minutes. Grind cashew adding enough milk to get a fine paste. Chop onion. Wash and pat dry chicken. Cut into bite sized pieces. Heat 1tbsn butter in a pan, add onion and ginger-garlic paste and fry till onion is golden brown. Add chicken and fry for while. Add tomato puree, Chilli powder, salt, turmeric, and masala, and cook for while. Add cashew paste along with milk. Cook covered till gravy is thickened and chicken is done. Top with remaining butter and remove from flame. Garnish with chopped coriander leaves. Serve hot with Roti or Nan.