Preheat the oven to 325°F (165°C). Spray a 9×5" loaf pan with cooking oil
In a bowl, mix together the flour, cornstarch, baking powder, and baking soda (for the plain banana bread dry ingredients).
In another bowl, mix together the flour, cocoa powder, baking powder, and baking soda (for the chocolate banana bread dry ingredients).
In a separate bowl, mash the ripe bananas with a fork until mostly smooth. Add the egg, vanilla extract, and salt. Mix well and set aside.
In the base of a stand mixer with a paddle attachment or using a hand mixer, beat the butter and sugar until light and fluffy, about two minutes. Scrape down the sides of the bowl and mix again for about 30 seconds.
Add the banana mixture to the butter and sugar mixture and beat until incorporated. Scrape down the sides of the bowl and mix again for about 30 seconds.
Divide the mixture into two separate bowls. In each bowl, pour the corresponding dry ingredients and fold together until just combined. In the bowl with the chocolate banana bread dry ingredients, mix in the mini chocolate chips until incorporated. You should now have two banana bread batters.
Grease and line a 9x5" glass pan with parchment paper.
Using a cookie scoop, alternate the batters in a checkerboard pattern. Use about 1/9th of the batter per scoop and create two layers with 9 scoops, alternating each scoop. The bottom layer should start and end with the same flavor, while the top layer should alternate the opposite way so chocolate is on top of banana and vice versa.
Once all the batter is added, use a toothpick to lightly swirl the batter together.
Top with the chocolate chips and bake for 50-55 minutes, or until a tester poked into the center comes out clean.
Remove from the oven and let the bread cool in the pan for 10 minutes. Then, transfer it to a wire rack to cool completely before slicing and serving.
