Lightly brush the insides of the molds with melted butter and set aside. Fill the lower bottom of the steamer with water and bring it to a boil.
In a bowl, combine egg yolks, condensed milk, and calamansi juice. Stir in a circular motion until smooth. Fill the molds with the flan mixture until ⅓ full. Arrange them in a single layer on the steamer basket.
Cover the steamer with a clean kitchen towel and steam the flan at low heat for 5 to 7 minutes or until it's no longer liquid but not fully set. Remove from heat and let it cool completely.
In a large bowl, sift together flour, sugar, baking powder, and salt. Add water and egg whites. Stir until just combined.
Add the puto mixture over the flan in the molds, filling them almost to the top. Steam on low heat for about 8 to 10 minutes or until the puto has risen.
Remove from heat and allow the leche puto to slightly cool. Gently remove them from the molds and serve.