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 the vanilla extract, 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. Divide 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.

TO MAKE THE PASTRY CREAM:

10. While the cakes cool, make the pastry cream. Put the egg yolks in a medium sized bowl and gently beat them together. Set aside.
11. Add the sugar, cornstarch and milk to a large saucepan and mix until combined.
12. Cook, stirring continuously, over medium-high heat until mixture begins to thicken and bubble.
13. Reduce heat to medium and simmer for 2 minutes. Remove from heat.
14. Add a little bit of milk mixture to the egg yolks and whisk together, then add egg mixture to milk mixture. This process ensures that you don’t cook the egg yolks too quickly and get scrambled eggs.
15. Place the pan back on the heat and bring to a light boil. Allow to boil for 2 minutes, stirring continuously.
16. Remove from heat and add butter and vanilla extract. Stir until smooth, then set in the fridge to cool completely.

TO MAKE THE FROSTING:

17. When everything has cooled, make the frosting. Combine the butter and shortening in a large mixer bowl until smooth.
18. Add about half of the powdered sugar and mix until well combined.
19. Add the banana extract, vanilla extract and 3-4 tablespoons of water or milk mix until well combined and smooth.
20. Add the remaining powdered sugar and mix until combined.
21. Add additional water or milk as needed to get the right frosting consistency and mix until well combined and smooth.

TO BUILD THE CAKE:

20. To assemble the cake, use a large serrated knife to remove the domes from the top of the cakes.
21. Place the first layer of cake on a serving plate or a cardboard cake round. Pipe a dam of frosting around the outside of the cake. I use Ateco tip 808 for the dam so that it’s tall.
22. Spread half of the pastry cream evenly on top of the cake layer, inside the dam, then smooth out the dam of frosting so that it’s all even.
23. Add the second layer of cake and repeat the dam and pastry cream filling.
24. Add the final layer of cake on top, then smooth out the frosting around the sides of the cake.
25. Frost the outside of the cake. Feel free to use my tutorial for frosting a smooth cake.
26. Finish off the cake by pressing the crushed graham crackers into the sides of the cake, then top with some swirls of frosting and banana chips. I use Ateco tip 844 for the swirls.
27. Refrigerate cake until ready to serve. Cake is best for 2-3 days.