Fill a 12-inch sauce pan halfway with water and set over high heat.
In a bowl, sift together cake flour, sugar, baking powder and salt. Set aside.
In another bowl, combine 1 1/2 cups water, ube extract, egg whites and fresh milk. Using a stand mixer, beat the mixture at low speed for 5 minutes.
Add the sifted dry ingredients and blend until smooth.
Lightly brush a mini muffin tin with vegetable oil.
Fill muffin tin ¾ full with the batter, about 1 tablespoon each.
Once the water in the pan is boiling, place the muffin tin in the pan and cover the pan with a lid.
Steam the puto for 4 minutes, remove the lid and sprinkle the tops with cheese, put the lid back on and then steam for 1 more minute.
Once the puto is cooked, remove from the pan and allow to cool at least 30 seconds before removing from the muffin tins.
Repeat steps 6 to 10 more times, making sure there is enough water in the pan each time.