Preheat the oven to 350°F / 175°C.
Line an 8x8-inch baking dish with parchment paper and spray with cooking spray. Set aside.
In a large bowl or the bowl of a stand mixer, cream together butter and brown sugar until light in color.
Add in the egg, vanilla extract, and salt. Mix until combined.
Mix in the all-purpose flour on low speed until just combined.
Fold mashed banana into the batter until fully incorporated.
Fold in the chopped chocolate chips, saving a few to sprinkle on top of the batter.
Pour the batter into the prepared pan and sprinkle additional chocolate on top.
Bake for 33-37 minutes until golden brown. Rotate the pan halfway through baking.
Let the bars cool in the pan on a wire rack. Once cool, cut into 16 squares.
Store the bars at room temperature for a few days or in the fridge for up to a week.
Tip: click step to get into cook mode