Steps

1. Combine all of the ingredients except the starch in a microwave-safe bowl. Cover tightly with plastic wrap. Microwave for about 2 minutes, stir, then microwave for another 4 minutes.
2. Alternatively, place the water in a medium saucepan (nonstick if possible) and bring to a boil over medium-high heat. Add the sugar, salt, corn syrup, and vanilla, and stir until dissolved. Add in the rice flour slowly, stirring constantly so it dissolves evenly. When it has combined into a smooth mass, take it off the heat.
3. Turn out the mass onto a work surface liberally dusted with the starch. Use a sharp knife or dough cutter to cut mass into about 20 pieces.
4. Roll each piece out into a flat disc. Place a spoonful of adzuki-bean filling in the center of each disc, then pull the mochi around the filling, pinching the edges together to seal.
5. Flip each mochi onto a plate dusted with potato starch. Serve at room temperature.