Preheat oven to 350°F.
Lightly grease a 9x13 inch baking pan with non-stick baking spray and set aside.
Beat softened butter in the bowl of a stand mixer fitted with the wire whip attachment for 30 seconds, then add in the sugars and beat until combined.
1/2 cup unsalted butter, softened, 1 cup granulated sugar, 1/3 cup dark brown sugar, packed
Add in the baking powder, baking soda, cinnamon, nutmeg and salt; beat until combined, scraping down sides of bowl occasionally.
1 1/2 tsp. baking powder, 1/2 tsp. baking soda, 1/2 tsp. salt, 1 tsp. cinnamon, 1/4 tsp. nutmeg
Beat in the eggs, mashed bananas, vegetable oil, and vanilla extract until combined.
3 overripe bananas, mashed (~1 cup), 2 large eggs, 1/3 cup vegetable oil, 1 tsp. vanilla extract
Turn off mixer, add in flour and mini chocolate chips. Fold until combined.
2 cups flour, 1 cup mini chocolate chips + 1/3 cup for garnishing on frosting
Pour batter into prepared baking dish, spreading evenly.
Bake for 23-25 minutes or until a toothpick inserted into the center comes out mostly clean. A few crumbs is OK (we don't want to over bake these beauties!) Cool bars completely.