Cupcake

1. Preheat the oven to 350F/180C line a muffin tin with cupcake liners, then set aside.
2. In a large clean bowl whip the egg whites until starting to foram then slowly drizzle in 2 tablespoons of sugar and continue to work on medium speed until stiff peaks form, then set aside.
3. In another clean bowl whisk together the flour, salt, baking soda, and baking powder, then set aside.
4. In another large bowl cream together the granulated sugar and butter until light and fluffy, make sure to scrape down the sides of the ball as you go to ensure everything is evenly incorporated. Then mix in the egg yolks and vanilla, again making sure to scrape the bowl.
5. In a bowl mash the banans so that no large chunks are left. Add to the creamed butter along with the sour cream, mixing until everything is evenly incorporated.
6. Gently fold in the dry ingredients until almost completely combined then fold in the egg whites, again until just combined. Over mixing will create more gluten and therefore a tougher crumb.
7. Scoop the batter into the cupcake liners, I like to use a large cookie scoop to help ensure each cavity is evenly filled to about 3/4 of the way. Bake for 13-16 minutes, or until a toothpick inserted in the center comes out with only moist crumbs no wet batter, or if you gently press the top of the cupcake it bounces back immediately.

Filling

1. Once the cupcakes have completely cooled use a knife to cut a hole out of the center, you want it to be fairly deep but not all the way to the bottom, carefully tear or slice the cupcake filling from the piece you took out. You basically just want the top part left to cover the ducle de leche filling. Place dulce de leche in a piping bag with no tip or a medium round tip to easily fill the holes of the cupcakes, then place the "lid" piece of the cupcake back on top. 
2. Swirl the frosting on top or use a knife to gently spread it. Using another small piping bag with a small tip or a plastic bag with the corner snipped off drizzle dulce de leche on top of the frosting then sprinkle with chocolate pieces. Best eaten the day they are made, store leftovers in the fridge.