Preheat oven to 350° F. Line two 9-inch round cake pans with parchment paper. Set aside.
In the bowl of a stand mixer fitted with a paddle attachment, cream butter until smooth. Add sugar and mix until creamy. Add eggs one at a time and mix until incorporated.
Add vanilla and mashed bananas. Mix until combined.
In a medium bowl, whisk together flour, baking soda, baking powder, cinnamon and salt.
In another bowl, mix together buttermilk and sour cream. NOTE: It will be chunky, that's okay!
Add ⅓ of dry mixture to mixing bowl, followed by half of buttermilk. Repeat and end with remaining ⅓ of dry mixture. Mix until flour is incorporated into batter.
Divide batter between the two prepared cake pans. Smooth out and level batter using mini off spatula or spoon.
Bake for 30-40 minutes, rotating pan halfway thru baking. Insert toothpick in center of cake to check for doneness. Toothpick should come out clean. Otherwise, bake for another 5-10 minutes or so.
Cool cake in pan for 5 minutes. Then loosen edges of cake with an offset spatula. Remove cake from pan and let cool to room temperature on wire rack.