Prepare puto molds by lightly brushing insides with melted butter. Set aside. Add water to lower bottom of the steamer and bring to a boil.
In a bowl, combine egg yolks, condensed milk, and calamansi juice. Stir together in a circular motion until smooth and well blended.
Fill molds to 1/3 full of the flan mixture. Arrange in a single layer on the steamer basket. Wrap steamer cover with clean kitchen towel.
Steam flan at low heat for about 5 to 7 minutes or until mixture is no longer liquid but not fully set. Remove from heat and allow to cool completely.
Meanwhile, in a large bowl, sift together flour, sugar, baking powder. and salt. Add water and egg whites. Stir together until just combined.
Add puto mixture over flan, filling molds almost to the top. Steam on low heat for about 8 to 10 minutes or until puto has risen.
Remove from heat and allow to slightly cool. Gently remove leche puto from molds.