Preheat the oven to 350 degrees F (175 degrees C). Line an 8x8-inch baking pan with parchment paper, leaving an overhang on 2 sides. Coat paper lightly with nonstick spray.
Place mashed bananas in a bowl. Stir in coconut sugar, egg, butter, milk, and vanilla extract until well blended.
Stir flour, cocoa powder, baking soda, and salt together in a separate, smaller bowl. Add to the banana mixture and mix until incorporated. Fold in chocolate chips.
Scrape batter into the prepared pan and smooth out the top.
Bake in the preheated oven until a toothpick inserted into the center comes out clean, 16 to 20 minutes. Lift out of the pan using the parchment paper overhang; place onto a cutting board. Slice into bars and let cool.
