Preheat your oven to 350 degrees, and grease 2 8-inch round cake pans. Set aside.

In a large bowl, whisk together the oil, brown sugar, and sugar until smooth. Add in the eggs, one at a time. Then, beat in the mashed bananas and vanilla extract.

In a medium bowl, stir together the flour, cornstarch, salt, and baking soda. Whisk the flour mixture into the sugar mixture in parts, alternating with the buttermilk. You want to start and end the process with the flour mixture. Stir until the batter is completely incorporated, about 1-2 minutes.

Divide your batter evenly into your prepared baking pans. Bake for 30-35 minutes, or until a cake tester comes out clean. Cool in the pans for 5-10 minutes before gently removing the cake layers and wrapping tightly in plastic wrap. Let the cakes cool completely before trimming the tops to create a flat surface on each layer for frosting. I find this easiest with a sharp serrated knife, and just go slowly as you create a flat layer.

In a large bowl over a double-broiler, whisk together the egg whites and sugar, and cook until the sugar has dissolved. The mixture should be at about 150 degrees.

Remove from the heat, and beat the mixture on medium speed until stiff peaks form. This will take about 10 minutes with a stand mixer, and a bit longer with a hand mixer.

Slowly beat in your softened butter. The mixture will become thin, and then it will look separated- keep beating. After 10-15 minutes of mixing, your frosting should come together into a smooth mixture.

Beat in the salt, vanilla extract, and caramel sauce until completely combined. Frost your cake as desired, using extra caramel sauce to decorate as desired. Slice and serve!