Preheat your oven to 375°F. Place the cast iron skillet inside the oven to preheat as well.
Prepare the banana leaf by briefly passing it over a flame until pliable. Line the hot skillet carefully with the leaf, allowing excess to hang over the edges. Brush the leaf generously with melted butter.
In a bowl, whisk together the rice flour, all-purpose flour, baking powder, and salt.
In a separate bowl, whisk the coconut milk, egg, sugar, and vanilla until smooth.
Add the wet ingredients to the dry and stir until just combined. The batter should be pourable but thick.
Carefully pour the batter into the prepared skillet. Sprinkle a small amount of sugar over the top if desired.
Bake for 25 to 30 minutes until puffed, deeply golden on top, and set in the center.
Remove from the oven and immediately brush with salted butter. Let cool slightly before slicing. Serve warm.