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Preheat the oven to 350 degrees F. Line an 8x8-inch baking pan with parchment paper, leaving an overhang on 2 sides. Coat paper lightly with nonstick spray.
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Place mashed bananas in a bowl. Stir in the sugar, egg, butter, milk, and vanilla extract until well blended.
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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 if using.
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Scrape batter into the prepared pan and smooth out the top.
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Bake in the preheated oven until a toothpick inserted into the center comes out clean, 16 to 20 minutes.
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Lift out of the pan using the parchment paper overhang; place onto a cutting board. Slice into bars and let cool.