Add the potato starch to the warm rice and mix well with a spatula until evenly coated.

Add the water and mix quickly, then microwave at 500W for about 5 minutes.

Remove from the microwave and let it steam for about 1 minute. Wrap a rolling pin or similar tool with plastic wrap, then carefully mash the rice mixture, being careful not to burn yourself.

Let it cool slightly, then shape into balls. Coat with sweet red bean paste or kinako as desired to finish.