Whisk rice flour, water, coconut milk, sugar, tapioca starch, yeast, vanilla extract, and salt together in a bowl until smooth. Cover with a lid. Let batter rest until air bubbles rise to the surface, about 2 hours. Stir well.
Grease cake molds with oil. Place a steamer insert into a saucepan and fill with water to just below the bottom of the steamer. Place cake molds on top and pour batter into each, leaving 1 3/4 inches of space at the top for rising. Bring water to a boil. Cover and steam until a toothpick inserted into a cake comes out clean, about 10 minutes.