Preheat oven to 350 degrees F / 175 C and grease an 8.5-inch x 4.5-inch loaf pan with non-stick spray.
Mash 1 cup of overripe bananas and combine with 1 Tbsp of freshly squeezed lemon juice. This prevents the bananas from browning and brightens up their flavor. Set aside.
Add 1/4 cup unsalted butter and 1/2 cup granulated sugar into a large bowl. Beat on a medium high speed for 1-2 minutes with either a hand mixer or a stand mixer with a whisk attachment. Mix until the mixture becomes lighter in color.
Mix in 1/4 cup vegetable oil and 1 large whole egg + 1 egg white on a medium speed. Scrape the sides and bottom of the bowl as needed with a rubber spatula.
Add in 1 1/2 tsp baking powder, 1/2 tsp baking soda, 1 tsp ground cinnamon, 1/2 tsp salt, and 1 3/4 cups of all-purpose flour. Mix on a low speed until just combined. Scrape the sides and bottom of the bowl as needed with a rubber spatula.
Pour in 1/3 cup of buttermilk and mix on a low speed until incorporated.
Mix in the mashed banana/lemon juice mixture on a low speed until they're fully incorporated into the batter.
Pour a third of the batter into the prepared pan, then spread half of the cinnamon sugar mixture spread in an even layer over the batter. Repeat this process to make two layers of cinnamon sugar, and top with the remaining batter.
Bake on the upper baking rack of your oven for 55-60 minutes, rotating the pan halfway through to make sure it bakes evenly.
Remove the loaf from the oven and let it cool in the pan for 10 minutes. Transfer to a wire rack to finish cooling.