Place the rice flour, sugar, salt, and yeast in a large bowl and mix well.
Add water a little at a time and mix well until you have a smooth batter without any lumps.
Leave the batter to sit in a warm place for 4 to 6 hours. The time needed for fermentation can be different according to the room temperature; the key is to use your intuition.
Gently stir the batter. (An hour before you intend to cook and serve the hoppers)
Thin the batter adding coconut milk a little at a time as needed.
Dissolve the baking soda in a tablespoon of warm water. Add and mix well. Let the batter rest for about 1 hour.
Heat the Appa pan over medium-high heat until it becomes hot.
Pour a ladleful of the batter into the center of the pan and tilt the pan to spread the batter thinly. A thin layer of batter should stick to the sides, with a thicker layer in the center.
Cover and cook for about 2 to 3 minutes over medium heat until the edges of the pancake are starting to brown, and the center is no longer runny.
Run around the edges with a palette knife and ease it onto a plate.