1. Grease three 8 inch round cake pans and preheat oven to 350°F (176°C).
2. Beat the butter and sugar until light and fluffy, about 3-4 minutes. Do not skimp on this step. Cream the butter for the full 3-4 minutes.
3. Add vanilla, sour cream and vegetable oil and mix until well combined.
4. Add eggs and egg white in two batches, mixing until well combined.
5. Combine dry ingredients in another bowl. Add half of the dry ingredients to the batter and mix until well combined.
6. Add the mashed bananas and mix until well combined.
7. Add remaining dry ingredients and mix until well combined.
8. Divid batter evenly between the cake pans and bake 24-26 minutes or until a toothpick inserted comes out with a few crumbs.
9. Remove cakes from oven and allow to cool for 3-4 minutes, then remove to cooling rack to cool completely.

WHEN THE CAKES ARE COOL, MAKE THE FROSTING:

1. Beat the butter and shortening together until smooth.
2. Slowly add about half of the powdered sugar, mixing until smooth.
3. Add the vanilla extract and the caramel sauce and mix until smooth.
4. Slowly add the remaining powdered sugar and mix until smooth.
5. Add the water (or additional caramel sauce), as needed, and mix until smooth.

TO LAYER AND FROST THE CAKE:

1. Use a large serrated knife to remove the domes from the tops of the cake layers so that they are flat.
2. Place the first layer of cake on your serving plate or on a cardboard cake circle.
3. Add 5-6 tablespoons of caramel sauce to the top of the cake layer and spread evenly over the cake.
4. Add about 1 cup of frosting on top of the caramel sauce and spread into an even layer.
5. Add the second layer of cake on top of the frosting.
6. Add another 5-6 tablespoons of caramel sauce to the top of the cake layer and spread evenly over the cake. Add another cup of frosting on top of the caramel sauce and spread into an even layer.
7. Top the cake with the remaining layer of cake.
8. Frost the outside of the cake with the remaining frosting. For help getting a smooth finish, refer to my tutorial on how to frost a smooth cake with buttercream.

TO FINISH OFF THE CAKE:

1. Place banana chips around the bottom edge of the cake.
2. Use the remaining caramel sauce to drizzle around the edges of the cake.
3. Use the remaining frosting to pipe swirls of frosting around the top edge of the cake. I used Ateco tip 844.
4. Place a few banana chips around the top of the cake.
5. Because of the caramel sauce, you may want to refrigerate this cake. I recommend storing it in an air-tight container. Cake is best served at room temperature. Cake is best for 2-3 days.