Peel the banana and slice about 1/2-inch thickness at an angle. Set aside.
Some banana leaves are thick and rigid, you may need to boil it briefly in a hot water for 1 minute or so until it's soft and pliable. It they are manageable without boiling, just wipe it clean with damp cloth on both sides
Place the rice flour, tapioca flour, and 300 ml of coconut milk in a mixing bowl and whisk until no lumps
In a medium sauce pan, add the rest of the coconut milk, sugar, vanilla extract, and salt over medium heat until the sugar has dissolved and the mixture come to a gentle boil. Remove from the heat. Stir this mixture into the rice flour mixture and continue to stir until batter is smooth. The batter should be thick but still spreadable. Set aside to cool down a little bit
Get your steamer ready by bringing water to a boil. Cut out about 6 x 6 inch square of banana leaves. Spread about 2 Tbsp of the batter in the middle of the leaves. Place one slice or two of bananas on top and fold the two sides up continue with folding the other two sides down, as if you are wrapping a parcel. Steam the nagasari for about 20 minutes. This cake can be served warm or room temperature
Kue Nagasari (Indonesian Rice Flour Cake with Banana) https://whattocooktoday.com/kue-nagasari.html