Preheat oven to 350 degrees. Line 3, 8" round baking pans with parchment paper. Butter and flour sides of pans (I use Baking PAM for this).

In a large bowl or base of stand mixer, beat butter, sugar and brown sugar on high speed until light and fluffy, about 3 minutes.

On medium speed, beat in eggs, one at a time, until well incorporated. Scraped down sides as needed. Then, beat in vanilla extract.

In a medium sized bowl, stir together flour, cornstarch, baking powder, baking soda, salt and cinnamon.

Alternate beating flour mixture and buttermilk into butter/sugar mixture, in about 3 to 4 additions. Start and end with flour mixture.

Divide batter evenly among prepared pans and cook until toothpick in center of pan comes out with just a few crumbs. About 27-30 minutes. NOTE: Tops are a dark golden brown when done.

Let cool at least 5 minutes before removing from pans.

In a large skillet, melt butter over medium heat. Then, add brown sugar, rum, vanilla, and cinnamon. Stir until mixture starts to bubble, about 2-3 minutes.

Add bananas and walnuts. Stir for about 2 minutes. Remove from heat and set aside.

In a medium sized sauce pan over medium-high heat, add sugar and water. Stir continuously until sugar melts (the mixture will turn grainy, turn to a salt looking substance, harden, and then melt). This will take about 10 minutes. Once sugar melts, stop stirring, turn heat to high and boil until mixture turns an amber color and has a nutty smell (watch closely to ensure you do not burn the sugar). Remove from heat.

Cut butter into small cubes and gently place in sugar. Whisk until smooth. Slowly pour in whipping cream, continuously whisking. NOTE: It is important that the butter and cream are not cold, if they are, it will make the caramel harden too soon.

Next, whisk in rum and vanilla. Set aside and let cool to room temperature. This will take about 2 hours.

In a medium sized bowl or base of a stand mixer, beat butter and shortening on medium-high speed until lightened, about 2 minutes. Then, beat in 1 cup of cooled caramel sauce.

Beat in powdered sugar, 1/2 cup at a time, until desired sweetness and consistency reached. NOTE: If you need to thicken the frosting but don't want it any sweeter, you can add cornstarch about 1/2 tbsp at a time.

Level tops of cakes with a knife or cake leveler. On top of two of the layers, pipe a border of frosting around the outside of the cake. Divide the bananas foster filling between the two piped cake layers.

Stack the two filled cake layers on top of each other. Place the final cake layer, leveled side down, on the top. Frost cake with a thin layer of buttercream. Place in fridge for about 10 minutes. Then, remove and finish frosting with buttercream as desired (I use a Wilton 2D tip to on the bottom and top of cake).

Decorate with cut banana and cinnamon as desired. Fill a squeeze bottle with caramel sauce and let drip over sides of cake.