Slice bananas thinly and add the banana slices to a mixing bowl. Add brown sugar and gently toss to coat the banana slices with sugar. Set aside to marinate for 10-20 minutes.
In another mixing bowl, combine tapioca starch, rice flour, salt and cinnamon powder. Then add 1/4 cup water and 3 tablespoons of water, mix well to create a smooth batter.
Reserve about 12-15 banana slices to decorate the top of the cake later, add the remaining banana slices to the batter. Gently fold the banana slices into the batter.
Pour the batter into a mold or pan which will fit inside your steamer (I use a loaf pan). Ideally, use a 4''x8'' loaf pan so that the cake will have a height of slightly over 1 inch. Place the reserved banana slices on top for decoration.
Set up a steamer and wrap a clean towel around the lid to prevent water condensation to drop on the cake (or wipe the lid clean every 10 minutes). Steam the cake over medium to medium-high heat until cooked through (about 25 minutes). The batter will turn translucent and the edges will start to pull away from the mold slightly. If you insert a toothpick, there should be no wet batter on the toothpick.
Remove the mold/pan from the steamer and let it cool down enough to handle. Take the cake out of the mold/pan. Cut into slices and serve warm with coconut sauce and toasted sesame seeds.