Soak the mung beans for several hours, then rinse well. Add them to a pot with 1 1/4 cups water (300 ml or more) and cook over low heat until the beans are soft and the water is absorbed. Let the beans cool.

Make the green batter: In a bowl, combine tapioca starch, rice flour, pandan leaf juice, coconut milk, and sugar. Stir until the flour and sugar are completely dissolved, then strain the mixture through a fine sieve for a smooth batter. Cover and set aside.

For the mung bean layer: Once the beans have cooled, transfer them to a bowl. Add tapioca starch, rice flour, sugar, and coconut milk. Mix well, then blend the mixture in a blender until smooth. Pour into a bowl and set aside for steaming.

Bring water to a boil in a steamer. Grease a 20 cm (8-inch) round cake pan and place it in the steamer for a few minutes to heat. Once hot, pour in a ladleful of the green batter to cover the bottom. Cover and steam for about 5 minutes. Open the lid, add a layer of the mung bean mixture, and wipe the lid dry each time you open it. Continue alternating layers, increasing the steaming time by 1 minute for each new layer. Finish with a green layer on top for a more attractive cake.

Test the cake with a toothpick; if it comes out sticky, steam a bit longer until the toothpick comes out clean.

Let the cake cool completely before cutting to prevent sticking. For a firmer texture and brighter color, you can place the cake pan in a bowl of ice water to cool.