In a 2- or 3-quart saucepan, whisk 2 tablespoons rice flour and ½ cup water together. Set over medium-high heat and cook, whisking constantly, until a sticky paste forms, 1 to 2 minutes. Set aside to cool.

In the bowl of a stand mixer fitted with the whisk attachment, combine remaining 1 ½ cups rice flour, sugar, yeast, baking powder, and salt. Mix on low speed until thoroughly combined, about 1 minute. Add cooled rice flour paste, coconut milk, and remaining ¾ cup water; increase speed to medium and mix, scraping down sides of bowl as needed, until a smooth, pancake-like batter forms, 5 to 7 minutes.

Remove bowl from stand mixer, cover with plastic wrap or a clean kitchen towel, and let stand at warm room temperature until the dough rises slightly and a few bubbles form on the top, 1 hour.

In a clean wok with a stacking bamboo steamer or another steamer setup, bring about 2 cups water to boil over high heat. (If using a wok with stacking bamboo steamers, you can cook 8 puto at once; if using a different steamer setup, you may only be able to cook 4 puto at a time.) Spray 2 ½-ounce silicone baking cups with cooking spray (the number of cups will depend on how many you can fit in your steamer setup at once). Fill baking cups with puto batter about three-quarters full and arrange them in your steamer. Stretch a clean kitchen towel across steamer, then close lid to hold towel in place (the towel will prevent condensation from dripping down onto the puto below). Steam until puto are puffed, firm, and have a pearlescent sheen, 20 minutes.

Line a serving platter with banana leaves, if using. Take out the first batch and place them on the platter. Cover puto with additional banana leaves (this imparts flavor).

Repeat with remaining batter and transfer to the platter when done (or store extra batter in the fridge for future batches). Serve.