Slice sponge cake into 3 layers using serrated knife, each about 1-cm thick.
Combine pandan juice, thick coconut milk, mung bean flour and custard powder, strain through a sieve into a cooking pot with thick base (to prevent burning).
Combine plain water (not hot water), sugar, salt and agar-agar powder in a sauce pot, bring to the boil, cook until agar-agar powder has melted.
Add agar-agar mixture to pandan mixture gradually while stirring. Cook mixture over medium-low heat until thickened. Cover and dip the cooking pot in hot water (to prevent mixture from setting).
Rinse 9-inch round baking pan with water, drain-off excess water (to help in unmoulding).
Pour 300 g of pandan layer mixture into baking pan, swirl to coat the sides. Top with 1 slice of cake, set aside for 3 minutes to allow pandan layer to slightly set. Repeat the same for 2nd and 3rd slice of cake.
Pour 300 g of mixture over 3rd slice of cake, set aside for 4-5 minutes. Pour the remaining mixture into the gap between the cake and baking pan, set aside until cooled.
When cooled, loosen the cake sides from baking pan with plastic knife.
Cover the baking pan, keep refrigerated for 4-5 hours or overnight until completely set.
Unmould and cut into slices.