Blend the rice flour, tapioca flour, sugar, coconut milk and salt. Pour this into a heatproof container and add the knotted pandan leaves.  

Steam this for about 10 minutes until it has thickened / jellified. Let it cool. 

Once it has cooled, stir to form a dough-like consistency.  

On a piece of banana leaf, transfer a spoonful of dough. Rest a piece of plantain or banana on the dollop of dough and then cover it with another spoonful of dough. Fold the banana leaf like an envelope to seal in the filling inside. 

Arrange the prepared Nagasari envelopes in the steamer. Steam for about 15 minutes or until the banana becomes soft and all the dough is cooked through.