In a large bowl whip the egg whites until starting to foam. Then slowly drizzle in 2 tablespoons of sugar and continue to mix on medium speed until stiff peaks form, then set aside.
Whip the Egg Whites
In another bowl whisk together the flour, salt, baking soda, and baking powder, then set aside.
Mix Dry Ingredients
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.
Cream the Butter and Sugar
Then mix in the egg yolks and vanilla, again making sure to scrape the bowl.
Add Egg and Vanilla
Add mashed bananas to the creamed butter along with the sour cream, mixing until everything is evenly incorporated.
Add Banana and Sour cream
Gently fold in the dry ingredients until almost completely combined then fold in the egg whites, again until just combined.
Mix all together
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.
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. 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.
Add Dulce De Leche
Swirl the frosting on top or use a knife to gently spread it. Drizzle dulce de leche on top of the frosting then sprinkle with chocolate pieces.