When ready to assemble, slice the sponge cake horizontally into 2 or 3 slices (depends on how many layers you want). Try to cut as flat and evenly as possible
Prepare custard:
Add hun kwe flour to the coconut milk and whisk to combine
In a heavy bottom pot or saucepan, add water, sugar, salt, and agar agar powder. Stir to combine. Let them sit for 20 minutes. The agar agar will dissolve better later
Put the pan on low heat, add butter, and cook until the agar agar dissolves. You shouldn't see anymore specks of agar agar. If they don't dissolve, your custard will not set later
Remove from the heat and add coconut mixture and stir to combine. Return it back to the stove and cook over low heat until the mixture thickens slightly and coats the spatula but still liquid and pourable. Stir in the cendol and sweetened red beans
To assemble:
Rinse the clean pan you use to bake the cake with water and then shake off excess
Pour in about 300 gr of custard (for 3-layer cake) or 400 gr for 2-layer cake. Gently put first layer of cake on top and just gently press the cake down a little bit. Pour in second layer of custard, then top with second layer of cake and pour the remaining custard if you are making 2 layer-cake. For 3-layer cake, top with the third layer of cake and finish off with the rest of the custard
Let the cake cools down at room temperature and then transfer to a fridge to let it cool for 5 to 6 hours or overnight
Serving:
Gently turn the cake upside down onto a serving platter. Wrap the side of the cake pan with a hot towel. It helps to release the cake. You can also roll the cake pan on its side gently and because of gravity, it will help to loosen the cake from the side and it will slide out later
The cake can be served as is or with some melted gula Melaka/gula Jawa
Storage:
It is best to finish the cake in about 3 days. I noticed that after 3 days, the custard starts to ooze out some liquid making the cake a bit wet. This is typical for agar-agar type of cake or custard