Preheat oven to 350°F. Prep the loaf pan by greasing it with oil or lining it with loaf liners and set aside. In a large bowl, mash the bananas and set aside.
In a large bowl, whisk mashed bananas, brown sugar, sour cream, melted and cooled butter, egg, egg yolk, and vanilla. Whisk until well combined.
1 1/4 cups bananas, 3/4 cups dark brown sugar, 1/3 cup butter, 2 tbsp oil, 1/2 cup sour cream, 1 egg, 1 egg yolk, 1 tbsp vanilla
Add in flour, baking soda, baking powder, cinnamon, nutmeg, cornstarch, and salt. Using a spatula, fold until a soft banana batter forms. Don't overmix!
1 3/4 cups flour, 1 tsp baking powder, 3/4 tsp baking soda, 3/4 tsp cinnamon, 1/8 tsp nutmeg, 1 tbsp cornstarch, 1/2 tsp salt
Add in mini chocolate chips. Mix in the mini chocolate chips.
3/4 cups mini chocolate chips
Evenly divide the batter between 8 loaves, they should be almost full. Using a cookie scooper helps big time to minimize the mess. Sprinkle streusel on top.
Bake the loaves at 350°F for 25-28 minutes. The banana bread loaves are ready when a toothpick inserted comes out clean.