Rinse and soak the dried red beans in water for at least 4 hours or overnight.
Drain the soaked red beans. Place them in a pot with boiling water and cook until very soft.
Drain most of the water from the cooked red beans, reserving a small amount. Mash about half of the cooked red beans.
Return the mashed and whole red beans to the pot. Add rock sugar and simmer, stirring until the sugar dissolves.
In a small bowl, mix potato starch with water to create a slurry.
Slowly stir the potato starch slurry into the red bean mixture. Cook, stirring constantly, until the mixture thickens to a paste-like consistency.
Pour the red bean mixture into a mold (e.g., an 8-inch square pan). Smooth the top and let it cool slightly, then refrigerate for at least 30 minutes to set.
In a small bowl, sprinkle gelatin powder over cold water and let it bloom for 5 minutes.
In a saucepan, combine coconut milk, water, and caster sugar. Heat over medium heat, stirring until the sugar dissolves. Do not boil.
Remove the coconut mixture from heat. Add the bloomed gelatin and stir until completely dissolved.
Let the coconut mixture cool down to room temperature, stirring occasionally to prevent a skin from forming.
Carefully pour the cooled coconut mixture over the chilled red bean layer in the mold.
Refrigerate the pudding for at least 2-3 hours, or until completely set. Once set, cut into squares or desired shapes and serve chilled.
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