Preheat your steamer over medium heat on the stove.
In a large mixing bowl, whisk the egg whites, water, and coconut milk until fully incorporated.
Sift the rice flour and baking powder over the wet ingredients. Use the whisk to mix all the ingredients together.
Pour the sugar into the batter and mix to combine.
Scoop the puto mixture into mini muffin molds to about ¾ of the way up. Place the molds into the steamer and cover with a lid.
Steam for 8 minutes and remove the lid to see if the puto is fluffy and firm. If you would like to add cheese, cut the cheese into small ½ inch slices and place on top of the puto. Cover with the lid and allow it to melt for about 30 seconds. Remove when the cheese has slightly melted.
Remove the puto from the steamer and let sit in the molds for 30 seconds to cool before removing them from the molds. Serve immediately.
Puto should be eaten immediately to experience the fluffy and airy texture. You can also keep them in the fridge for a few days. Do not keep them on the counter overnight because they are moist and tend to mold quickly.IMPORTANT: I included ingredients by weight because this is more accurate for baking. If you don't have a scale and must use volumetric measurements, I've included those in the recipe above too, but please note the volumetric measurement numbers will not change if you scale up or down the servings made in this recipe and you will need to do your own math to figure it out.