Preheat oven to 350°F In a large frying pan, combine all the ingredients for the caramel. Mix well with a spoon, then place over medium heat. Allow sugar to caramelize to a light golden brown.
Immediately pour the caramel into 9 Inch x 13 Inch x 3 Inch baking pan.
Hold pan with kitchen towels and tilt the pan so the caramel completely covers the bottom. Set aside.
Prepare the Flan:
In a large mixing bowl, combine the flan ingredients except the lemon or dayap rind. Strain, using a fine strainer like a piece of cheesecloth. Stir in grated lemon or dayap rind. Pour the flan mixture into the caramel lined baking pan. Set aside.
Prepare the cake:
In a large mixing bowl, blend the first 8 ingredients for the cake. Beat with a wire wisk 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 caster sugar while continuing to beat at medium speed until egg whites are stiff but not dry.
Quickly but thoroughly fold in egg yolk mixture into the beaten egg whites.
Carefully pour the cake batter into the pan with the flan. Do not tap the pan.
Bake for 1 hour and 25 minutes using a water bath. Allow the cake to cool in its pan together with the water bath.
Run spatula around the cake to loosen it and invert to a serving platter. Chill well before serving.