Preheat the oven to 350℉. Prepare the loaf pan with parchment paper or grease it and lightly coat with flour. In a medium bowl, mash the bananas with a fork. Set aside.
Add the butter to a large glass mixing bowl or microwave-safe bowl. Melt in the microwave in 30-second increments until the butter is melted. Add the sugar, milk, sour cream, vanilla extract, and eggs to the bowl and whisk.
In a separate medium bowl, sift together the baking soda, salt, and flour. Gradually add the dry ingredients to the bowl of wet ingredients, making sure to scrape the sides of the bowl to incorporate all the ingredients. Mix until just combined. There should be no dry spots in the batter. Finally, fold in the chocolate chunks.
Pour the banana bread batter into the prepared pan and bake for 70-80 minutes. The banana bread will be done when the top is golden brown and if you stick a wooden toothpick in the center of the loaf no wet batter sticks to the toothpick (moist crumbs are fine). Allow it to cool in the pan for about 20 minutes before transferring to a wire rack. Slice and serve with butter!