Prepare the red bean filling by rinsing 1 cup dried adzuki beans thoroughly. Soak in cold water for 4 hours or overnight. Drain and add to a pot with 3 cups fresh water. Bring to a boil, then reduce heat and simmer for 60-90 minutes until beans are completely soft and mashable.

Drain the cooked beans and transfer to a food processor. Add 1/2 cup sugar and blend until smooth. Transfer the paste to a non-stick pan and cook over medium-low heat, stirring constantly, for 10-12 minutes until the mixture thickens and pulls away from the sides. Let cool completely, then divide into 12 equal portions and roll into balls. Refrigerate.

In a large mixing bowl, combine 2 cups sweet rice flour, 1/3 cup sugar, and 1/4 teaspoon salt. Gradually add 1 cup water while stirring to create a smooth batter with no lumps.

Prepare a steamer by bringing water to a rolling boil. Line a steaming tray with parchment paper or a damp cloth. Pour the batter into a shallow heat-proof dish that fits in your steamer. Steam over high heat for 25 minutes until the dough becomes translucent and fully cooked.

While still hot, transfer the steamed dough to a work surface dusted generously with potato starch. Knead the dough with a wooden spoon or spatula for 3-4 minutes until it becomes smooth and elastic. Be careful as it will be very hot.

Divide the dough into 12 equal portions. Flatten each portion into a circle about 3-4 inches in diameter. Place a red bean ball in the center and carefully wrap the dough around it, pinching the seams closed and rolling gently to create a smooth ball.

Toast 1/2 cup soybean powder in a dry pan over medium heat for 2-3 minutes until fragrant. Let cool slightly. Roll each filled mochi in the toasted soybean powder until fully coated. Serve at room temperature.