In a medium bowl, sift the flour, baking powder, baking soda and salt. Set aside.

In a stand mixer fitted with the paddle attachment, beat the butter, oil, and sugars on medium-high speed until pale and fluffy, about 5 minutes. Lower the speed to medium and add the eggs, one at a time, mixing until each is fully incorporated before adding the next (about 30 seconds). Add the vanilla.

With the mixer on low, add the dry ingredients alternating with the buttermilk mixture into the creamed mixture, starting and ending with the dry ingredients. Mix until just incorporated and remove bowl from stand mixer. Using a large stainless steel whisk, stir/whisk the mashed banana into the mixture--you can use use some strength and blend well, ensuring all of the ingredients are well mixed. (If you don't have a large stainless whisk, you can fold the banana mixture in using a rubber spatula until incorporated.)

Divide the batter evenly between the pans, using a kitchen scale for the best accuracy. (For this recipe, you'll have about 15 ounces of batter in each of the four 6-inch pans or 20 ounces of batter in each of the three 8-inch pans). 

Let pans cool on wire rack for 10 minutes, then invert cakes onto racks, gently, peeling away parchment rounds. Let cool completely. 

When the cakes are completely cooled, level the cake layers if necessary

The cake layers can be made up to a week in advance. If you plan to make it in advance, cover each cake layer with two layers of plastic wrap. When you're ready to frost your cake, take the cake layers out of the freezer an hour before you decorate.

Combine the butter and cream cheese in the bowl of a stand mixer fitted with the paddle attachment and beat until combined, about one minute.

With the mixer on low speed, gradually add the sifted powdered sugar, followed by the banana emulsion and salt.

Increase the speed to medium and beat the frosting for 5 minutes, until light in color and texture.

Continue to mix by hand using a wooden spoon to smooth out air bubbles. 

In the bowl of an electric mixer fitted with the paddle attachment, combine the butter and peanut butter. Beat on medium-high speed until smooth, about 45 seconds.

Add the powdered sugar to the bowl and mix in, slowly at first until incorporated, then increasing the mixer speed to high. Blend in the vanilla extract, salt, and cream. Continue to whip on medium-high speed until very fluffy, about 4-5 minutes, scraping down the sides of the bowl as needed.

Continue to mix by hand using a wooden spoon to smooth out air bubbles.

Mix the sugar, water and corn syrup in a small saucepan.

Bring the mixture to boil over medium heat, but DO NOT STIR! (Stirring encourages crystallization). Let the mixture cook until it turns to a golden brown/amber color. It happens quickly, so keep an eye on it.

Remove the caramelized sugar syrup from the heat and very slowly stir in the cream, pouring it in little by little, to temper the cream and sugar syrup. If you pour it in all at once, your sugar will turn into hard candy. You can start adding more and more as you go along.

Once the cream is mixed in, add the salt and vanilla. Stir to combine and then set aside to cool to room temperature before using.    

After you've leveled each cake layer with a cake leveler or serrated knife, place the first cake layer, top side up, on a cake board.

Cover the cake layer with about 1/2 cup banana frosting.

Drizzle with salted caramel and place another cake layer on top.

Apply a thin layer of frosting around the entire cake to lock in the crumbs. Freeze for 10 minutes.

After the crumb coat is set, continue to frost the cake with the peanut butter frosting and add the caramel drip.