Divide the koshian paste into 8 portions, then shape into balls using two spoons. Set aside on a plate in the refrigerator. Combine the rice flour, sugar, salt and water in a microwave-safe bowl. Heat in the microwave oven for 2 minutes at 600 watts. Take out and mix with a wet spatula, then heat for another 40 seconds to 1 minute. The dough should now be translucent. Incorporate the adzuki beans.
Dust a large tray with enough potato starch to completely cover the surface, then place the dough on the tray. Dust the surface of the dough with potato starch. Divide into 8 using a dough-cutter. The dough needs to be worked while it is still hot, but be careful not to burn yourself. Take the balls of koshian out of the refrigerator. Stretch a piece of the dough with your hands until it is big enough to wrap around a ball of koshian. Place the piece of dough in the hollow of your hand, dust off any excess potato starch and put a ball of koshian in the middle. Close the dough over the koshian, pinching the edges together to seal. Place the stuffed rice cake, seam side down, on a plate dusted with potato starch. Make the others in the same way.
— Traditionally, kuromame (black soya beans) are used for these, but they can be difficult to find outside Japan, so I’ve replaced them with adzuki beans.
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