Prepare Oven and Pan:
Position oven rack in middle and preheat to 375°F (190°C).
If using banana leaves, rinse and pat dry. Hold over medium-high gas flame for 15 seconds per side until soft and pliable.
Line 8-inch round pan with banana leaves, overlapping slightly. Brush with 1/2 oz melted butter. (Alternative: grease pan and line bottom with parchment paper)
Mix Ingredients:
In a medium bowl, whisk together white rice flour, sweet rice flour, baking powder, and salt for 30 seconds.
In a large bowl, whisk together 1 oz melted butter, sugar, coconut milk, and egg for 1 minute until sugar dissolves and mixture is smooth.
Add dry ingredients to wet ingredients. Whisk for 45 seconds until smooth and lump-free.
Bake:
Fold batter gently once or twice with spatula. Pour into prepared pan and spread evenly.
Bake for 10 minutes until edges start setting and center is slightly jiggly.
Remove from oven. Distribute macapuno evenly over partially-baked cake.
Return to oven and bake for additional 12 minutes until puffed and edges are lightly golden. Test with toothpick—should come out with moist crumbs.
Finish:
Switch oven to broil. Brush cake top with remaining melted butter and sprinkle generously with sugar.
Broil for 2 minutes, watching constantly, until top is golden brown and macapuno caramelizes slightly. Position rack 6 inches from broiler element.
Transfer to wire rack and cool for 10 minutes. Run knife around edges, invert onto rack, then flip right-side up onto serving platter.
Peel away banana leaves or parchment. Serve warm.