Preheat the oven to 350 °F and prepare the caramel In a large frying pan, combine sugar, water, and cream of tartar. Mix well with a spoon, and then place over medium heat. Do not disturb the mixture once it's over direct heat. Allow sugar to caramelize to a light golden brown
Immediately pour the caramel into a 9'' x 13'' baking pan (3 inches high)
Using kitchen towels, tilt the pan so as to encourage the caramel to completely cover the bottom of the pan. Set aside.
Combine the flan ingredients in a large mixing bowl strain, using a piece of cheesecloth or fine strainer. Stir in 2 teaspoons of grated dayap or lemon rind. Transfer the flan mixture to the caramel lined baking pan. Set aside.
Prepare the cake. Blend the first 8 ingredients for the cake in a large mixing bowl. Beat with a wire whisk until smooth and lump free. Set aside.
In another large mixing bowl, using an electric mixer, beat the egg whites together with the cream of tartar until frothy. Gradually add the white sugar while continuing to beat at medium speed until the egg whites are stiff but not dry.
Quickly but thoroughly fold in the egg yolk mixture into the beaten egg whites Carefully pour the cake batter into the pan with the custard. Do not tap the pan.
Bake for one hour and 25 minutes using a water bath ( place cake pan in a larger cake pan that you fill with water) Allow the cake to cool in its pan together with the water bath. Unmold carefully and chill well before serving.
