Mix the sugar, soy sauce and water in a saucepan, and heat until the sugar fully dissolves, stirring so it doesn't burn.
Add the rice flour, and mix well until it forms a cohesive dough.
Transfer the dough to a microwave-safe container, and microwave for 30–40 seconds, then take it out and mix thoroughly. Repeat until the mixture is evenly cooked through.
Mix in the walnuts.
Spread the mixture evenly onto the prepared baking sheet, about 5 cm thick; let it cool a little first if it's too hot.
Dust the top with more starch, and let cool completely.