Preheat the oven to 350°F. Grease a 15x21-inch baking sheet and set aside.
In a small container, combine 1 cup milk and 1.5 tablespoons of apple cider vinegar to make homemade buttermilk. Set aside while you work on the rest of the cake.
In a separate small bowl, mash together the bananas using a hand mixer, or by hand. You will need a total of 340 grams or 1.5 cups of mashed banana. Please be sure to measure as banana sizes vary.
In a separate small bowl, whisk together 2.5 cups of flour, 1.5 teaspoons of baking powder, ½ teaspoon of baking soda, 1 teaspoon of kosher salt, and ¾ teaspoon of cinnamon (optional). Set aside.
Using a handheld electric mixer or a stand mixer with the paddle attachment, beat 1 stick of unsalted softened butter, 1.25 cups of white sugar, and ½ cup of brown sugar on medium speed until beginning to come together—about 3-4 minutes. Make sure to scrape the bowl a couple of times to ensure everything is incorporated. It will be a bit lumpy at this point.
Add the 3 eggs, 1 tablespoon of vanilla, and ¼ cup of avocado oil. Beat on medium-high speed until completely incorporated. Next, mix in the mashed bananas. Scrape down the sides and up the bottom of the bowl to ensure everything is completely incorporated.
Sift in about a third of the dry ingredients, and mix with the mixer on low until just combined. Then, add in about a third of the homemade buttermilk mixture and combine gently. Continue alternating the dry ingredients and the buttermilk and mix on low speed until each addition just until incorporated. Do not overmix, or the cake will become dense. The batter will be thick, and it may have a few lumps.
Pour the batter into the prepared baking dish and smooth the top using an offset spatula or rubber spatula. Bake in the preheated oven for 25-30 minutes or until a toothpick inserted into the center comes out clean.
Once the cake is done, remove it from the oven and let it cool completely in the baking pan on a wire rack.