Preheat oven to 350℉ with a rack in the center of the oven. Prepare sheet pan with cooking spray and parchment paper. Spray parchment too!
In a medium bowl, whisk flour, baking powder, baking soda, and salt together.
In a stand mixer with the paddle attachment (or a hand mixer), combine butter and sugar and beat on medium-high until light, fluffy, and doubled in volume (2-3mins). Scrape sides with rubber spatula as needed.
Reduce mixer to low and add the egg, then increase speed to medium-high and beat until light and fluffy (1-2min).
With mixer on low, slowly pour in buttermilk, oil, and vanilla and increase speed to medium high and beat until the batter is almost white and doubled in volume (~5min).
Reduce mixer to low and add bananas a handful at a time and beat until broken into pieces and evenly distributed throughout the batter.
Add dry ingredients gradually and beat until just combined. Scrape bottom and sides of bowl and mix on low for an additional 30 seconds.
Add 3/4 of the chocolate chips and mix for ~30 seconds to evenly distribute them throughout the batter.
Pour batter into pan. Sprinkle the top with the remaining chocolate chips. Bake for 35-40 minutes, until golden brown and a toothpick inserted comes out with a few crumbs attached.
Add frosting if you'd like, but I ate it plain!