Heat the butter in a frying pan. Fry onions, ginger, cummin seeds till light brown in color. Add the mashed potatoes, garam masala salt and chilli powder and mix well. Let the mixture cool while you prepare the basic paratha dough. Make a ball, roll it in flour and roll out thin. Put 2 tsp of the potato stuffing in the middle. Pull up the dough from all sides and pinch at the centre. Dip the ball in dry flour and roll out. Cook on a hot griddle, following the same method as for parathas. Serve hot.