Dissolve a cube of bullion chicken stock in 3 cups of warm water and set aside. Wash and blanch baby bak choy in salt water. Deep in cold bath, drain and set aside. Wash and soak dried shrimps in warm water for 30 mins. Drain dried shrimps and set aside. Retain the shrimp water. Steam yam for 25 mins. Heat oil in wok and sauté dried shrimps till it is soft. Add yam, fish sauce and chicken stock. Reduce heat to medium and let it simmer till yam is very soft and gravy reduced. Add baby bak choy and mix well for about 2 mins. Transfer to serving plate and serve hot.