In a bowl, combine rice flour, sugar, baking powder, and salt. Whisk until distributed.
Add water and coconut milk. Stir until smooth and well-blended. Cover with film and refrigerate overnight.
Lightly spray insides of puto molds with non-stick cooking or lightly brush with melted butter. Pour puto batter into molds until 3/4 full.
In the bottom part of the steamer, add water and bring to a boil. Arrange filled puto molds in a single layer on the steamer basket. Place steamer basket over the bottom part. Wrap lid of the steamer with cheesecloth or any cotton material (to prevent condensation from dripping onto puto) and cover steamer.
Steam for about 15 minutes or until a toothpick inserted in center comes out clean.
Remove from the heat and allow to slightly cool and gently remove puto from the molds.