Preheat the oven to 325 degrees F. Coat a quarter sheet baking pan with nonstick spray, line it with parchment paper and spray again. 

Mix the flour, salt and baking powder in a large bowl. Set aside.

In a bowl of a standing mixer fitted with the paddle attachment, beat the butter, oil and granulated sugar together until light and fluffy. Add the egg whites, one at a time, beating well after each addition. Add the vanilla.

With the mixer on low speed, beat in the flour mixture and buttermilk alternately, beginning and ending with the flour. Scrape down the bowl and mix until incorporated.

Bake for 20 minutes, and then test to see if it's done using a toothpick inserted into the center of the pan. Only a few crumbs should be one the toothpick. May need to cook it for an additional 3-5 minutes, depending on your oven.

Mix the pudding mix, sweetened condensed milk and milk in a bowl until well blended, about two minutes. Transfer the mixture to a shallow dish, cover, and refrigerate until set, at least 2 hours.

Whip the heavy cream until stiff peaks form. When the pudding is set,  fold it into the whipped cream in small batches. Carefully fold in all of the pudding until it is completely blended with the cream.

Cover and refrigerate until you're ready to build the cake. This step can be done a couple days in advance.

Combine the pudding mix and milk in a bowl and blend. Mixture should be thin, like milk. 

Best to make right before assembly.

Mix the water, corn syrup and granulated sugar together in a very small saucepan. Bring the mixture to a boil over medium heat, but do not stir. Cook until the mixture caramelizes into the beautiful golden brown color. It happens quickly once the sugar starts browning, so don't walk away from the stove during this step.

Remove the caramelized sugar syrup from the heat and mix in the cream. If the cream is not room temperature, add it SLOWLY, while mixing. Add a bit more and mix that in. Continue mixing in the cream a little bit at a time at first, and then you can start adding more and more as you go along.

Mix in the salt and vanilla extract. Stir to combine. Set the caramel aside and let it cool completely before using, or store it in an air-tight container until ready to use.

For the Toasted Graham Cracker Crumbs

Preheat oven to 350 degrees F.

Crush the graham crackers in a food processor or by hand.

Add the crumbs, sugar and salt in a bowl and whisk to combine.

Add the melted butter and stir until all the crumbs are wet.

Spread evenly on a baking sheet lined with parchment paper and bake for 15 minutes. Let cool completely and set aside.

In a bowl of a standing mixer fitted with the whisk attachment, combine the cream and sugar. Whisk until stiff peaks form.

Place a piece of parchment paper on the counter. Invert the cake onto it and peel off the parchment from the bottom of the cake. Use a 6" cake ring to stamp 3 circles in the cake (or two circles and two halves). 

Place the cleaned cake ring on the center of a sheet pan lined with parchment paper. Use one strip of acetate paper to line the inside of the cake ring.

Put one layer of the cake (or two halves) inside the ring.

Dunk a pastry brush in the banana milk soak and slather it on the cake, giving it a nice milk bath.

Spread one-third of the banana cream pudding in an even layer over the cake.

Spread sliced bananas evenly over the pudding (if using). 

Sprinkle 1/2 cup of the toasted graham cracker crumbs on top of the bananas, followed by a heavy drizzle of the salted caramel.

Carefully spread about 1/2 cup of the whipping cream on top.

Gently tuck in another strip of the acetate paper between the cake ring and the top quarter inch of the first acetate strip, high enough to support the height of the finished cake.

Set a cake round on top of the whipped cream and repeat steps 4-8.

Set the final cake layer onto the whipped cream and cover the top of the cake with the remaining banana cream pudding. Sprinkle it with graham cracker crumbs, drizzle with salted caramel and decorate with whipped cream.

Transfer the sheet pan to the freezer and freeze for at least 12 hours. About 4-5 hours before you're ready to serve, remove the cake from the freezer and transfer to the refrigerator to finish thawing.

Serve chilled.