For the custard :
6 eggs (yolks and whites)
1 cup sweetened condensed milk
1 cup skim milk (the ready-to-drink kind)
1/4 cup white sugar
1/4 teaspoon finely grated lemon rind
For the cake:
3 egg yolks
1/4 cup minus 1 tablespoon white sugar
3/4 cup cake flour
1/2 teaspoon baking powder
1/4 cup skim milk
3 egg whites
1/4 cup minus 1 tablespoon white sugar (no, this is not a typo — you need two measures of the same amount of sugar)
Place all the ingredients for the custard in a bowl.
Mix until the sugar is completely dissolved. Do not beat as you do not want air bubbles in the mixture.
Place a strainer (you can use a piece of muslin) over the baking pan and pour in the custard mixture.
Sift together the cake flour and baking powder.
Place the 1/4 c. minus 1 tbsp. of sugar and egg yolks in a bowl.
Beat until smooth and lemon colored.
Add the flour mixture and milk alternately, mixing after each addition.
After all the flour and milk have been added, mix until the batter is smooth.
In another bowl, beat the egg whites until foamy. Add the second 1/4 c. minus 1 tbsp. of sugar little by little while beating continuously. Continue beating until the egg white mixture is stiff.
Now you mix the egg yolk and egg white mixtures together. Remember that the reason you beat the egg white to stiff peaks is to create all those tiny air bubbles. It is these bubbles that will give volume and the light texture to your cake. Work carefully so as not to break them.
Add half of the egg white mixture to the egg yolk mixture and, using a rubber scraper and the cut and fold technique, blend the two mixtures together. Remember to scrape the sides and bottom of the bowl.
Then, add this blended mixture to the remaining half of the egg white mixture, and continue cutting and folding until the mixture is well blended and the color is even. Again, remember to scrape the sides and bottom of the bowl. No streaks of white nor yellow should be visible.
Pour the cake batter over the custard. Unless you overworked the mixtures and the tiny bubble burst, the cake batter should float on the custard.
Using a spatula, smoothen the top of the batter carefully. It doesn’t have to be perfectly smooth. This is an upside-down cake and so the top becomes the bottom, and out of view, when you serve the cake. Just make sure that all the sides are sealed with the cake batter so that the custard does not boil over it during baking.
Place the cake pan in a larger pan half-filled with hot (not boiling) water. Bake in a preheated 350F oven for 50 minutes to an hour. After 50 minutes, insert a toothpick at the center of the cake. If it comes out clean, the cake is done. If not, bake a few minutes longer, testing every five minutes or so.
That’s how the cake looks when it comes out of the oven. The top is nicely browned. It’s tempting to invert and serve the cake at this point — BUT DON’T! The custard is still soft and you’ll ruin the cake if you try and serve it at this stage. Cool the cake — COMPLETELY. Place it on the counter with a screen cover. After an hour or two, you can continue cooling it in the fridge if you’re really in a hurry to serve it.
When the cake has cooled, take a plate that can hold the cake and place it upside down on top of the baking pan. With one hand under the baking pan and the other hand on top of the plate, invert the cake onto the plate. I advise you to do this over the sink because there is always a chance that the caramel will spill.
In case the layers of cake and custard aren’t too distinct in the photo above, see the one below.