Wash and soak the two types of rice and Urad dal separately (in separate vessels) for at least 3 hours in plenty of water
First grind the Urad dal to a fine paste and remove it - this helps in making the Sannas fluffy. Next, grind both types of rice together to a fine thick batter (not as coarse as Idli batter). Try to use as little water as possible to grind. It should be of dosa batter consistency but not too thick. Transfer to a wide, deep pan large enough to accomodate batter that will double during fermentation.
Prepare the yeast solution by mixing the yeast and sugar in a bowl of tepid water. The sugar helps the yeast to dissolve faster. Allow to stand for 10 minutes, the yeast will ferment and turn frothy. Stir to ensure that all the yeast granules have dissolved and what you have is a thick solution.
Mix the yeast solution into the batter until completely incorporated. Add sugar to taste (batter can be mildly sweet like appams - but it's upto you how sweet you want them). Add salt to taste (around 1 level tsp) Mix well and cover with a thin muslin cloth and place it undisturbed in a warm place to ferment for about 2 hours (during really hot weather, the batter will ferment in just about 1-1/2 hours)
The batter would have doubled - for fluffy Sanna do not stir the batter or it will go flat. Place sufficient water in the steamer (tondor) and bring the water to a boil. Keep ramekins greased and ready. Pour batter into them half full. Place these ramekins into the steamer, cover the lid and steam for 15-20 minutes. Remove the lid carefully so as to prevent the vapour from falling on the Sanna (making the surface slightly soggy) and the steam from burning your hands/face. Carefully remove the ramekins and place them face down on a large plate. Allow to cool (to speed up this process if you need the ramekins for the second batch, place cold wet towels over them to speeden up the cooling process).
Once cool, remove carefully and serve ! enjoy!