Soak red beans in water for at least 4 hours, or ideally overnight, until the beans absorb enough water.

Drain the soaked beans and transfer them to a pot. Cover with fresh water and bring to a boil over high heat until the beans are completely soft, which may take 30-1.5 hour depending on the beans and on how long you soaked them. Test if the beans are well cooked by "squeezing test": squeeze a bean if it can be easily crushed by your finger.

Once the beans are cooked, drain any excess water. Use a blender or food processor to blend beans with a small amount of water until smooth. Then, add sugar and cook over low heat, stirring constantly, until the paste thickens to your desired consistency. Let it cool completely before using.

Combine all the dry ingredients (glutinous rice flour, cornstarch, and optional sugar or cornstarch) in a bowl. Gradually pour in the water, mixing well until the mixture is smooth and lump-free.

Cook the mixture. Stove method: Pour the mixture into a non-stick pan and cook over low heat, stirring constantly. The dough will thicken and turn shiny within 2–4 minutes. Steaming or microwave method: Pour the mixture into a heatproof dish, cover with plastic wrap or a lid, and cook for 15–20 minutes. You can use a toothpick or skewer to check if it's done: if it comes out clean, the dough is fully cooked. Remember to stir the dough midway through microwaving or steaming for even cooking.

Once cooked, let the dough cool slightly, then knead it until smooth and elastic. To prevent sticking, use a small amount of water, oil, or roasted glutinous rice flour on your hands and work surface. If desired, add coloring or flavor during kneading. Then, divide the smooth dough into small, evenly sized pieces.

You should use a hemisphere mold to achieve the best mochi shape. Sprinkle a little roasted glutinous rice flour on top to prevent sticking, then flatten the dough and place it over the mold. Next, add the red bean paste (you can use a piping bag to fill the mochi if the filling is soft for easier filling), then gather the edges to seal it.

Finally, trim off the excess dough and gently sprinkle some more roasted glutinous rice flour on. Transfer the mochi to trays/ paper cups. Repeat these steps until all the dough and filling are used.