The cake is usually baked in a llanera (flan mold), which is oval in shape. I don't have the pan and so I use one 8-inch round pan. This recipe can fit into 2 8-inch llanera pan
Preheat oven to 350 F (180 C) for a conventional oven. For a convection oven, preheat to 330 F (165 C)
Prepare a tray/baking pan larger than your round baking pan and deep enough to hold about 1 inch of hot water bath for baking later. Do not grease the pan
Prepare caramel sauce:
Put sugar and water in a saucepan over medium heat. Bring to a boil and then lower the heat to let it simmer gently until the syrup turns amber in color, like the color of a honey. Take care not to cook until the color is too dark as the caramel turns bitter
Quickly but carefully pour the caramel into the pan and swirl the pan at the same time to evenly cover the base of the pan. Set aside to let it solidify
If you use two llanera pans, divide the caramel between the two pans
Prepare the leche flan batter:
Combine eggs, lightly beat them with a whisk to loosen a bit. Add the condensed milk, evaporated milk, vanilla extract, and salt. Whisk to combine, but don't whisk too vigorously because it creates too many air bubbles that will make your flan porous. Set aside. We will strain this batter later to make sure there are no lumps from the egg whites
Prepare the cake batter:
Make sure you have hot water for the water bath baking later
Separate the egg yolks from the whites when they are still cold. It's easier to do so when they are cold. Let the egg whites come to room temperature.
Puree the banana, egg yolks, sugar, milk, and vanilla extract until smooth. Pour into a mixing bowl. Sift in the flour, ground cinnamon and salt. Combine into a smooth batter
Place the egg whites in a mixing bowl. Whisk over medium speed using a whisk attachment for one minute until foamy and then add vinegar and whisk for another minute. Increase the speed to high and add sugar in 3 batches and beat until the meringue is glossy and stiff. You can always stop halfway and check. When you lift the meringue up with the whisk attachment, the meringue should stand up stiff and just bent slightly at the tip but not moving. If you turn the bowl upside down, the meringue won't drip. This is a stiff peak and you can stop whipping. Do not overwhip the meringue or it's going to break
Scoop 1/3 of the meringue into the cake batter. Use a whisk to gently fold the meringue into the cake batter. Cut through the middle and fold over. Then the next 1/3 and do the same
Switch to a spatula on the last 1/3 and use a swipe down and fold over motion to make sure everything is properly combined, especially at the bottom and the side of the mixing bowl
Assembling:
Gently pour the leche flan mixture over a sieve/strainer, to catch any lumps, onto the baking pan or llanera that you have poured caramel on top earlier. We don't want to create too many air bubbles
Next, gently pour the cake batter over the leche flan layer. Don't worry about the layer will get mixed together. They won't as long as you pour the cake batter gently. Smooth the top as needed. Gently tap the cake pan on the counter 2-3 times to pop any large bubbles
Bake in a water bath:
Place the pan inside another larger tray/baking pan. Pour hot water into the tray, about 1 inch high
Carefully place this inside the oven, middle rack, and bake for about 70 minutes or until the top of the cake is dry to touch and springs back when you gently press on it. If you insert a skewer inside the sponge cake, it should come out clean with a few crumbs is okay as long as it's not wet
Cooling:
Remove from the oven and put on top of a cooling rack. Let it cool down inside the pan completely at room temperature. Cover with a cling wrap over the pan and then refrigerate for at least 4 hours, overnight if possible, before cutting the cake
When ready to serve:
Remove the cake from the fridge. Use an offset spatula to loosen the side of the pan. The caramel at the very bottom has turned into a sauce now. Put a large round plate on top of the pan and then carefully but quickly flip the whole thing upside down. Gently remove the pan and voila, there you have it.
How to store:
Simply cover the cake tightly with cling wrap and keep in the refrigerator for 3-4 days