Peel 2 ripe bananas and place in a microwave-friendly bowl. Microwave for 1 minute. Then use a fork and mash-up the bananas very thoroughly.
Take out a mixing bowl. Place in the glutinous rice powder (1.25 cups). Place in Salt (1 teaspoon). Mix the two together. Then add-in the mashed bananas. Mix everything. Add in a few drizzles of warm water until the dough starts to clump and turns into a ball. Keep kneading until the dough doesn't stick to your fingers. (Add more water if it falls apart). Set aside for now.
Cut the crust and outer layer off your castella cake. Then use a cheese grater and grate your slice of cake into small sprinkles.
Now, peel your 3rd ripe banana. Cut it into thin small pieces.
Back to the dough. Snap-off a quarter-sized piece. Stretch it thin with your fingers (we don't want it too thick). Now place one or two banana pieces in the middle. Close it with your fingers and roll it in back and forth in the palm of your hands. This should form it into a ball. Repeat until you run out of dough.
Fill a pot with water. Bring it up to boil. Then place your dough balls inside. Use a chopstick or kitchen utensil to give the balls a slight nudge (once in a while) so they don't stick to the bottom of the pot.
Once all of the balls rise to the surface, let it boil for another 1 minute. (This will ensure that the dough is fully cooked through).
Drain the water. Place the balls on your cooking board and let them completely cool-down. Let it drip off excess water. Once they're completely cool, roll each ball into your castella sprinkles. Toss them well and get them evenly coated.
Serve immediately! With a cup of tea or coffee :)