Preheat your oven to 350°F (177°C). Grease and lightly flour a 9x13 inch baking pan (or line with parchment paper). In a medium bowl, mash the 4 overripe bananas with a fork until smooth with only small chunks remaining. Set aside.
In a separate bowl, whisk together the flour, baking powder, baking soda, cinnamon, and salt. Set aside.
In a large bowl or stand mixer fitted with the paddle attachment, beat the softened butter, granulated sugar, and brown sugar together on medium-high speed for about 2–3 minutes until the mixture is light, fluffy, and completely combined.
Add the eggs one at a time, beating well after each addition. Mix in the vanilla extract, mashed bananas, and buttermilk. (Note: The batter may look slightly curdled or separated at this stage; this is completely normal).
Turn the mixer to low speed and slowly add the dry ingredient mixture to the wet ingredients. Mix just until combined and no large flour pockets remain. Do not overmix, or the cake will become dense.
Pour the batter into your prepared 9x13 pan and smooth the top with a spatula. Bake for 30–35 minutes, or until a toothpick inserted into the center comes out clean. Remove from the oven and let the cake cool completely in the pan on a wire rack before frosting.
While the cake cools, make the frosting. In a large bowl, beat the softened cream cheese and butter together on medium speed for 2 minutes until completely smooth and creamy.
Gradually add the powdered sugar one cup at a time, followed by the vanilla extract. Beat on high speed for another 1–2 minutes until silky and fluffy.
Spread a thick, even layer of the cream cheese frosting over the completely cooled banana cake. Top with chopped walnuts or a dusting of cinnamon if desired. Slice and serve!