Add salt to rice flour and mix well. Add boiling water, gradually, to the rice powder and mix using a wooden spatula/spoon. Once the mixture cools down a bit, knead with your hands to make a soft and pliable dough. Cover the dough with a wet kitchen towel to prevent it from drying out.
Combine grated coconut and grated jaggery.
Clean the banana leaves with a wet towel. Place the cleaned leaves over gas flame, flip the sides in between, to wilt the leaves slightly. This will prevent the leaves from breaking while folding. It's better to use leaves without the middle stem.
Grease the wilted leaves lightly with coconut oil or ghee. Place big lemon size balls of dough on the leaves and spread it thin using your finger tips. Make sure to dip your finger tips in water while doing this.
Place some filling on one side of the rolled out dough and fold from the other side and press lightly.
Once the water starts boiling in the steamer, place the adas and cook on high flame for 8-10 mins. Open the steamer and see whether the ada is separated from the leaves, then it's cooked.
You can also cook it on Dosa pan. heat the dosa pan on medium-high heat, once the tawa is really hot, reduce flame to lowest and place the ada on tawa. Cook both sides for 3-4 mins, each
Notes
You can also add crushed cardamom to the coconut jaggery mixture. The qty of jaggery can be adjusted to suit your sweetness level. If you are steaming, make sure not to crowd the steamer with adas. If you are making large qty, steam ada in batches. Also the cooking time for ada varies depending on the thickness of adas and how much adas you've placed in the steamer. You can also use banana in the filling along with coconut and jaggery or use Chakka Varattiyathu (jackfruit preserve) as the filling.