In a hot stock pot coated with oil, season the chicken and brown all sides. Put chicken aside. In the same stock pot, remove chicken fat, leaving only a coating of oil and saute onions, garlic and ginger. Caramelize well, then add the curry powder. Mix quickly for 2 minutes making sure not to burn the curry powder. Add back the chicken, banana bay leaves and chicken stock. Check for seasoning. Bring to a boil and simmer slowly for 1 1/2 to 2 hours. During the last 30 minutes of cooking, add the yams. Serve on white rice or with toasted pita bread.