Use baking powder, not baking soda
These two leaveners are not interchangeable. Baking powder already contains the components it needs to create lift, while baking soda requires enough acid in the recipe to work properly.
Double-acting baking powder is particularly useful because it provides lift during mixing and again when the batter becomes hot.
Mix gently after adding the flour
Once the wet and dry mixtures come together, use a spatula and stop as soon as no large pockets of dry flour remain.
Small lumps are less damaging than overmixing. Excessive stirring develops gluten and can make the steamed cakes tough.
Treat the resting time as brief
Rest the batter for 10 to 15 minutes as written, but avoid leaving it for several hours. Baking powder becomes less predictable when mixed batter is held too long.
When steaming several batches, gently stir from the bottom before refilling the molds without vigorously beating out the trapped air.
Watch the batter level
Fill each mold approximately three-quarters full. Overfilled molds can rise over their edges, touch neighboring cakes, or produce uneven tops.
Use the same dispenser, ladle, or spoon for every mold so the puto finish steaming at approximately the same time.
Start with a fully heated steamer
Bring the water to a strong boil before placing the molds inside. Beginning with a cold steamer makes the cooking time less predictable and can reduce the initial rise.
Check the water level between batches and add hot water when necessary. Do not allow the steamer to boil dry.
Manage condensation from the lid
Water dripping from the lid can leave wet spots or dents on the puto. If your steamer produces heavy condensation, wrap the lid with a clean kitchen towel and secure it away from the flame or heating element.
Open the lid carefully so trapped water does not fall onto the finished cakes.
Avoid opening the steamer too early
The batter needs uninterrupted heat while its structure sets. Opening the lid during the first several minutes releases steam and can cause the puto to collapse.
Check only near the end of the expected cooking time.
Use the toothpick test carefully
Insert the toothpick into the cake rather than through the melted cheese. It should come out clean or with only a few moist crumbs.
Do not continue steaming much longer once the cakes are done because their edges can become dry or rubbery.
Cool briefly before unmolding
Freshly steamed puto is delicate. Allow it to cool just enough for the sides to firm up before removing it from the molds.
Release the edges gently rather than pulling the cakes out by the cheese topping.