Preheat oven to 350 F. In a medium bowl, whisk together flour, salt and baking powder. Set aside.
In a large mixing bowl, beat the egg yolks with a wire whisk until it is frothy and pale. Add the sugar and beat until incorporated. Add the oil, vanilla and water. Stir well. Gradually add the flour mixture while stirring, until the mixture is smooth.
In the bowl of a stand mixer (or use a hand-held mixer), beat the egg whites until frothy. Add the lemon juice and continue beating until the mixture forms firm peaks. To test, turn the bowl over carefully. The meringue (beaten egg whites) should not drip.
Fold the meringue into the yolk batter in three additions. Gently combine with a folding motion of a rubber spatula until the mixture is uniform in color. Scoop the batter into large muffin molds, or mamon molds until they are 2/3 full.
Bake the mamon at 350 F for 22-25 minutes, or until the top is golden and a toothpick inserted at the center of a mamon comes out clean.
Once the mamon are cool enough to handle, fill the centers with the yema. Transfer the yema into a piping bag fitted with a filling tip (one that is round ended). Poke the center of a mamon gently and push filling into the inside until some of the yema is oozing out. Repeat with the rest of the mamon. Pipe remaining yema over the mamon.
Combine cornstarch and milk in a medium saucepan and stir until smooth. Add the condensed milk and egg yolks and stir until combined. Cook mixture over medium heat while stirring until it is thick and spreadable. Let cool. This can be done up to two days ahead. Just store the yema in a covered container in the fridge.