Rinse and soak the Hulled split mung beans for 1–2 hours (or use them unsoaked and cook a bit longer). Drain, then steam or simmer the beans until very soft, about 20–30 minutes.
While hot, mash or blend the cooked mung beans into a smooth paste, then stir in about half of the Coconut milk and some of the Granulated sugar to taste; cook gently until the mixture thickens into a spreadable paste. If the paste seems loose, add a tablespoon or two of Rice flour to help bind and cook until it holds its shape.
In a large bowl, whisk together the Tapioca starch and the remaining Rice flour, then add the remaining Granulated sugar and a pinch of Fine salt. Gradually whisk in the rest of the Coconut milk and enough Water to make a smooth, pourable batter without lumps.
Divide the batter into two equal bowls. Stir the Pandan juice into one bowl to create the fragrant green batter; leave the other bowl plain for the white/yellow coconut layers.
Prepare a steamer with simmering water and line or lightly grease an 8-inch round pan with a little Neutral oil. Bring the steamer to a steady simmer so it’s ready before you begin layering.
To build the cake, pour a thin layer (about 3–4 tablespoons) of the pandan batter into the pan, spread evenly, and steam uncovered for 6–8 minutes until just set. Remove from the steamer briefly to pour the next layer.
Spread a thin layer (about 2–3 tablespoons) of the mung bean paste over the set pandan layer, then steam 6–8 minutes until the mung layer firms. Next pour a thin layer of the plain coconut batter over the mung paste and steam 6–8 minutes.
Repeat the sequence (pandan batter → mung paste → plain batter) until the pan is filled, finishing with a top layer of batter. For each thin layer steam 6–8 minutes; after the final layer steam the whole cake for 20–25 minutes to ensure it cooks through.
Turn off the heat and let the cake cool in the steamer for 10 minutes, then remove and cool completely to room temperature before unmolding. Chill for easier slicing if desired.
Run a knife around the edges, invert onto a cutting board, and slice into squares or wedges. Serve at room temperature or slightly chilled — the cake keeps in the fridge for several days.
