In a stand mixer fitted with the paddle attachment, beat together butter, sugar, and oil on medium speed for 3-4 minutes until fluffy. Scrape down the sides and the bottom of the bowl occasionally. Beat in the eggs one at a time, beating well after each addition. Add mashed bananas, buttermilk, and vanilla extract and beat until combined. With the mixer on low speed, add half of the dry ingredients and beat just until combined, then add the rest of the dry ingredients and beat just until combined. Do not overmix, the less you mix, the lighter the cake will be.
Pour batter into prepared pan and bake for 35-45 minutes until a toothpick inserted into the center of the cake comes out clean. Allow to cool to room temperature. Frost cake with cream cheese frosting.
Store cake in the refrigerator in an airtight container for up to 5 days.