Preheat oven to 375℉ and grease a 13" by 9" cake pan
In a large bowl, whisk together flour, baking powder, baking soda, cinnamon, and salt. Set aside.
Using a separate bowl, beat vegetable oil and sugars together until smooth.
Beat in eggs, vanilla, buttermilk, and milk, until well combined.
Pour wet ingredients into the flour mixture and stir until just combined.
Fold in the mashed bananas.
Pour batter into the prepared pan.
Bake in preheated oven for 30-35 minutes or until the center is baked through. Use a toothpick or fork to test the center of the cake, if it comes out clean it is ready.
Allow to cool in pan before cutting into squares and serving.