Soak rice, fenugreek seeds and both the dal together for 2 to 3 hours. Soak rice flakes seperately for an hour. Grind everything together to smooth and thick paste. Mix salt to that. Allow to ferment for 16 to 20 hours. Next day one hour before pouring dosa mix rawa and sugar. Spread dosa as thin as possible. Fry on one side only with enough oil till crisp. Keep a tbsp of potato masala inside and dot with little butter. Fold into a triangle and serve hot with coconut chutney and sambar.