For caramel : Combine 3/4 cup of sugar and 1/4 cup of water in a pan. Cook in low heat until it turns amber color. Don't stir, just swirl. Transfer to 10 inch pan and let it cool. Set aside.
For custard : Combine 5 whole eggs, 1 can condensed milk (390g), 1 can evaporated milk (370ml), 1 tsp vanilla extract (or lemon zest). Mix well. Then strain two times using a fine sieve (or lemon zest). Mix well. Then strain two times using a fine sieve to make it smooth. Pour on to the cooled caramel. Set aside.
For the cake : combine 1 1/2 cup flour, 1 1/2 tsp baking powder and 1/2 tsp salt. Sift to remove lumps. Add 1/2 cup sugar and mix well. In a separate bowl, combine the dry and wet ingredient then mix well. Set aside.
For meringue. Beat 4 egg whites until foamy. Add 1/4 tsp cream of tartar. Beat at high speed. Add 1/4 cup sugar gradually. Beat until stiff peaks.
Fold the meringue to the cake batter in three parts. Do not overmix.
Then pour the cake mixture on the top of the custard mixture gently.
Cover with foil to avoid water drips while steaming. Then, steam for 50 seconds to 1 minutes depending on the pan size in pre-heated ovenated 180 C. Test the cake using the toothpick test.
Remove foil and let it cool before unmolding. Once completely cooled, scrape the side with knife to unmold.