In a medium-large microwave-safe bowl, whisk together the mochiko, sugar, and water. Whisk until the mixture is smooth and there are no more clumps remaining. Clean the sides of the bowl by scraping down any residual batter.
Cover with plastic wrap and microwave for 1 minute.
Using a wet spatula, mix the mixture well. Recover with plastic wrap and microwave for another 1 minute. You will repeat this a third time for an additional 1 minute. Only repeat afterwards (in 30 second increments) if the dough isn't yet ready. It should be thoroughly sticky and translucent. Different microwaves have different power levels so adjust accordingly.
Generously cover your work surface (either a sheet pan or large cutting board that will fit in the fridge) with potato starch.
Pour out the hot mochi onto the potato starch, coat the mochi dough and your rolling pin with more potato starch and roll out into a thin rectangle.
Let cool and set in the fridge for 15 - 30 minutes.
Remove the mochi from the fridge and cut into 8 pieces (These mochi will be chubby!)
Working one at a time to prevent melting, remove one of your frozen filling balls from the freezer and unwrap.
Use a pastry brush to remove the excess potato starch from your dough. Place the filling ball in the centre of your mochi wrapper. Fold the sides up and pinch shut. To help you get the proper shape and make sure everything is well sealed, wrap your mochi in plastic wrap and twist closed. Place in the freezer, and repeat with your remaining mochi.
After waiting at least a few hours for the mochi to set, you can defrost your mochi at room temperature (it will take a few hours) before enjoying.
Note: Don't store your mochi in the fridge, it ruins the texture. Storing your mochi in the freezer and then defrosting as desired, maintains the freshness of the mochi!