1.Put 3/4 cup (150 grams) 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. Carefully pour the caramel into an 8" cake pan and swirl the pan at the same time to evenly cover the base of the pan. Set aside to let it solidify.
2.In a large bowl add 4 eggs, and briefly whisk them. Add the condensed milk, evaporated milk, 1 teaspoon vanilla extract, and 1/8 teaspoon salt. Whisk gently to combine, set aside.
Make sure you have hot water for the water bath baking later
3.Separate 4 egg yolks from the whites when they are still cold. Let the egg whites come to room temperature. In a blender add banana, 4 egg yolks, 2 1/2 teaspoon sugar, milk, and 1 teaspoon vanilla extract then puree until smooth. Pour mixture into a large mixing bowl.
4.Sift in the cake flour, cinnamon and 1/2 teaspoon salt. Combine into a smooth batter. Preheat the oven to 350 degrees and place a small pot of water to boil. Take out a 9" cake pan.
5.In the bowl of an electric mixer with a whisk attachment add 4 egg whites. Whisk over medium speed for one minute until foamy and then add vinegar and whisk for another minute. Increase the speed to high and add 5 1/4 tablespoons (65 grams) sugar in 3 batches and beat until the meringue is glossy and stiff. When you lift the meringue up with the whisk attachment, the meringue should stand up stiff and just bend slightly at the tip. Do not over whip the meringue.
6.Scoop one third 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 same for the next third. Switch to a spatula on the last third and use a swipe down and fold over motion to make sure everything is properly combined, especially at the bottom and sides of the mixing bowl
7.Gently pour the leche flan mixture over a sieve, into the baking pan with caramel. We don't want to create too many air bubbles. Next, gently pour the cake batter over the leche flan layer. Smooth the top, as needed. Gently tap the cake pan on the counter 2-3 times to pop any large bubbles
8.Place the pan inside another larger 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 toothpick inside the sponge cake, it should come out clean with a few crumbs is okay as long as it's not wet
9.Remove from the oven and put the cake on top of a cooling rack. Let it cool down inside the pan completely at room temperature. Cover with plastic wrap and refrigerate for at least 4 hours, overnight if possible.
10.Remove the cake from the fridge. Use an offset spatula to loosen the side of the pan. Drip the bottom briefly with hot water. 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 allow the flan and cake to come out. Serve immediately.