DON'T GREASE YOUR MOLD
Put ten cups of water in a large steamer and let the water boil.
Separate the egg whites from the yolks. Set aside the yolks for the Leche Flan. See notes for the recipe link.
Put the egg whites and sugar in a large mixing bowl. Mix on high until the sugar is incorporated. Add the butter and milk.
Sift the flour, baking powder, and salt on top of the bowl and mix using a spatula. Then, use the mixer. Don't overmix, so use the spatula to crush solids left in the batter
Fill the plastic puto molder with about half of the batter and steam for twenty minutes and steam until cooked through. Fifteen minutes into steaming put the salted egg on top and cook for five more minutes.
I do not use a toothpick method in this because usually, the puto still comes out uncook even when the toothpick comes out clean; so, using a pointy knife, gouge out and peek if there is still soft batter underneath or in the middle.