Melt the butter in a small saucepan. Skim off the foam, then slowly pour the clear melted butter into a large nonstick skillet set over moderately high heat, stopping when you reach the milky residue. Add the mustard seeds and cook until they begin to pop, about 30 seconds. Add the onion, ginger, garlic and coriander and cook over moderate heat until the onion is softened, about 5 minutes. Add the chile, ground dried mango, garam masala and potatoes and cook for 5 minutes, stirring and mashing gently. Remove the skillet from the heat, stir in the cilantro and season with salt and pepper.