DON'T GREASE YOUR MOLD.
Put 6 cups of water in a large steamer and the water boil.
In an electric mixing bowl, put the egg, milk and butter.
Combine the flour, salt, sugar ,baking powder in a medium sized bowl and mix it with the wire whisk to combine. Add the dry ingredients to the egg whites, milk and butter and mix well until batter is smooth.
Fill the plastic puto molder about half of it and for 13-20 minutes and steam until cook through. I do not use a toothpick method in this because usually, the puto still come out uncook even when the toothpick comes out smooth; so using a pointy knife, gouge out and peek if there is still soft batter underneath or in the middle. Then continue steaming.
When the puto cheese is cook line it with cheese on top.