Preheat oven to 350°F. Line an 8x8 baking pan with parchment. Set aside.
Using a stand mixer or handheld beater, beat together sugar and butter until fluffy and pale in colour. Scrape down the sides and bottom of the bowl.
Add in sour cream and egg and beat until combined. Scrape down bowl again.
Mix in the mashed banana.
In a separate bowl, whisk together flour, baking soda, baking powder and salt.
Add the dry ingredients to the wet ingredients and mix until you can still see some streaks of flour.
Mix in the mini chocolate chips and stir until there are no more streaks of flour and chocolate chips are evenly distributed. Make sure not to over-mix.
Transfer the batter to the prepared 8x8 baking pan.
Bake for 40-50 minutes, or until a toothpick inserted into the middle comes out clean. A few crumbs on the toothpick is fine, you just don’t want it to come out covered in raw batter.
Remove from the oven and allow to cool slightly in the pan. Then, remove from the pan and allow to cool completely on a wire rack.