Preheat and Prepare Pans: Heat the oven to 350°F. Grease three 8-inch round cake pans and line the bottoms with parchment paper.
Mix Dry Ingredients: In a bowl, combine flour, baking soda, baking powder, cinnamon, and salt.
Cream Butter and Sugars: In a mixer, beat butter, oil, granulated sugar, and brown sugar for 4 minutes until fluffy.
Add Eggs: Beat in the eggs one at a time, fully mixing each before adding the next.
Add Flavors: Mix in the vanilla, then the sour cream and mashed bananas.
Combine Wet and Dry: Add half the dry mixture, then the buttermilk, followed by the rest of the dry ingredients. Mix just until combined.
Bake: Divide the batter between the pans and bake for 25 minutes, or until a toothpick comes out clean. Cool for 10 minutes, then transfer to a cooling rack. Let cool completely before frosting.