Wash potatoes and remove any eyes.
Wrap each potato individually in plastic wrap and place them on a microwave-safe plate. Microwave the potatoes on high power (1000W) until tender, about 3-6 minutes (depending on the number and size of the potatoes).
Remove the plastic wrap from the potatoes and let them cool until they are easy to handle. While the potatoes are still warm, peel off the skins and put them in a bowl. Mash them with a fork, potato masher, or pestle until smooth.
Mix the starch with water in a small bowl until smooth. Add the dissolved starch and salt to the bowl (salt is not necessary if you plan to serve this dish with a sweet soy glaze), mix well, and shape the mixture into ⅘ inch (2 cm) thick rounds, about 1.4 oz (40 g) each.
Heat oil in a pan and cook the shaped potatoes over medium heat until browned on both sides.
(For the sweet soy glaze) Mix the starch with water in a small bowl until smooth. In a small pot, combine sugar, soy sauce, and water, and bring it to a gentle boil over low heat. Gradually add the dissolved starch to the pot while stirring constantly to thicken it and avoid lumps.
(To serve with the sweet soy glaze) Serve on plates and drizzle with the glaze.