Saute onion and ginger in butter for 5 minutes. Add coriander, turmeric, cumin, mace, salt, pepper and chile. Simmer for 1 minute. Add tomatoes and bring to a boil. Reduce heat to very low and stir in yogurt. Simmer for 5 minutes. Add potatoes and mung beans. Simmer for 10 minutes. Serve over rice, sprinkle with cilantro.