Preheat oven to 350 °. Prepare the baking pans by greasing them or lining them with parchment paper.
Peel and chop the bananas. Add them to a bowl and mash them with a potato masher or fork.
Allow the butter, eggs, buttermilk, and sour cream to come to room temperature. Your ingredients must be at room temperature so that they will incorporate properly when you are mixing the batter.
Measure out all of your ingredients. Sift all of the dry ingredients together: flour, baking soda, baking powder, salt, and cinnamon.
In the bowl of a stand mixer fit with a paddle attachment, or in a large bowl with a hand mixer, cream together the butter and your sugar until light and fluffy, approximately 3-4 minutes, halfway through stop and scrape down the sides and bottom of the bowl. This process helps make your cake rise and keeps it very light and tender.
On low speed, add the eggs one at a time into the creamed butter and sugar mixture, and mix until they are completely absorbed into the mixture before adding the next egg. During this process, periodically scrape down the sides of the bowl, and once all eggs are added, scrape the sides and bottom of the bowl to make sure all the eggs are mixed into the mixture.
Add the vanilla, mashed bananas, and sour cream into the mixture until combined.
Alternate between the dry ingredients and buttermilk as you add them to your batter. Begin by adding 1/4 of your flour mixture and mix until just absorbed, then add 1/3 of the buttermilk until it is incorporated. Continue this process, alternating between adding dry and wet ingredients to the batter. Always start and end with the dry ingredients.
Pour the batter evenly into the cake pans and place them in the oven. Bake for 20-25 minutes. Since ovens can vary, check for doneness by inserting a toothpick into the center of the cake – it should come out clean.
Allow the cakes to cool for 10 minutes in the pan, then carefully remove them from the pan and place them on cooling racks. Once the cakes have cooled, proceed to frost them with your favorite frosting. We recommend either a chocolate fudge frosting or a cream cheese frosting.
