Preheat oven to 325°F. Place muffin liners in a muffin tin and coat with cooking spray.
Stir together the flour, salt, baking powder and 1 c. of the sugar in a large bowl. Make a well in the middle of the flour mixture and add the oil, egg yolks, orange peel, vanilla, orange juice and water. Mix with your hands until there are no lumps. (Be sure to wash your hands after handling raw eggs!)
In a separate large mixing bowl, beat the egg whites with the cream of tartar until foamy. Gradually add the remaining 1/2 c. sugar, continuing to beat until soft peaks form. Lift your beater or whisk straight up; the egg whites will form soft mounds rather than a sharp peak. Fold the flour mixture into the egg whites. Pour the resulting batter into the prepared muffin cups until each is about halfway full.
Bake about 30 minutes, or until a toothpick inserted into the center comes out clean. While the cakes cool in the tin for 10 minutes, brush each one with melted butter. To top them off in the traditional Filipino way, sprinkle a little cheese on top of each one.
Remove to cool completely on a wire rack. Then enjoy!