Preheat the oven to 350℉/180℃. Line a 9x9 baking tray with parchment paper and set aside.
In a bowl, add in flour, baking soda, cinnamon, cornstarch, and salt. Stir to combine and set aside.
2 cups all-purpose flour, 1 tbsp cornstarch, 1 tsp baking soda, 1 tsp cinnamon, 1/4 tsp salt
In a large bowl, add in butter, white sugar, and brown sugar. Mix with an electric mixer on medium speed for 2-3 minutes until well combined.
1/3 cup dark brown sugar, 1/3 cup white sugar, 1/2 cup unsalted butter
Add in the eggs and vanilla. Continue mixing on medium speed until well combined. The mixture may curdle but that is normal.
2 large eggs, 1 tbsp vanilla
Add in the bananas, oil, and sour cream. Mix together for another minute on medium speed, scraping the bowl when needed.
1/4 cup full-fat sour cream, 2 tbsp oil, 1 1/2 cups ripe bananas
Add in the dry ingredients. With a spatula, gently fold until almost all the flour streaks have disappeared.
Toss in the chocolate chips, and continue to stir until all the flour streaks disappear and the chocolate chips are evenly distributed. Do not over-mix!
3/4 cups mini chocolate chips
Transfer into the prepared pan and spread evenly. Sprinkle extra chocolate chips and white sugar for a sweet top (optional).
Bake for 45-55 minutes or until a toothpick inserted in the middle comes out clean.
Allow to cool, slice, and enjoy!