Preheat your oven to 325 degrees F. Spray three 8-inch round cake pans with nonstick spray, line the bottoms with parchment paper, and then spray again. Set aside.
In a medium bowl, whisk together the flour, baking powder, baking soda, and salt. Set aside.
In a stand mixer fitted with the paddle attachment, beat the butter, oil, and sugars on medium-high speed until pale and fluffy, about 5 minutes. Lower the speed to medium and add the eggs, one at a time, mixing until each is fully incorporated before adding the next (about 30 seconds). Add the vanilla. Continue to mix on medium-high speed for an additional 2 to 3 minutes. The mixture should be smooth and fluffy.
With the mixer on low, add the dry ingredients alternating with the buttermilk mixture into the creamed mixture, starting and ending with the dry ingredients. Mix until just incorporated and remove bowl from stand mixer. Stir in the bananas.
Divide the batter evenly between the three pans, about 20 ounces in each.
Carefully smooth the batter with small offset palette knife, and bake until a toothpick comes out with a few moist crumbs on it, about 35 to 40 minutes.
Let pans cool on wire rack for 15 minutes, then invert cakes onto racks, gently, peeling away parchment rounds. Let cool completely.
When the cakes are completely cooled, level the cake layers if necessary
The cake layers can be made up to a week in advance. If you plan to make it in advance, cover each cake layer with two layers of plastic wrap. When you're ready to frost your cake, take the cake layers out of the freezer an hour before you decorate.