Preheat the oven to 425°F (220°C). Line all 12 cups of a standard muffin pan with paper liners. Lightly grease the liners if they are not labeled nonstick.

Place the peeled bananas in a large mixing bowl. Mash with a fork until mostly smooth. A few small pieces are fine, but large chunks can create wet pockets inside the baked muffins.

Add the melted butter and brown sugar to the mashed bananas and whisk until evenly combined. Add the eggs, sour cream, and vanilla, then whisk until the mixture looks smooth and no streaks of egg remain.

In a medium bowl, whisk together the flour, baking soda, baking powder, cinnamon, and salt. Break up any compacted flour so the leavening is distributed evenly.

Add the dry ingredients to the banana mixture: Fold gently with a flexible spatula until only a few small flour streaks remain. The batter should be thick and scoopable rather than pourable.

Reserve 2 tablespoons (about 20 g) of the chocolate chips for topping. Fold the remaining chocolate chips into the batter just until evenly distributed. Stop mixing as soon as no dry flour remains.

Divide the batter evenly among the 12 prepared cups. Each cup should be nearly full. Sprinkle the reserved chocolate chips over the tops.

Bake at 425°F (220°C) for 5 minutes. Without opening the oven door, reduce the temperature to 350°F (175°C).

Continue baking for 15 to 18 minutes. The muffins are done when the tops are rounded and golden, the centers spring back lightly, and a toothpick inserted into the center comes out clean or with a few moist crumbs. Avoid testing directly through a melted chocolate chip.

If the tops darken before the centers finish baking, place a loose sheet of aluminum foil over the pan and continue baking for another 2 to 3 minutes.

Place the muffin pan on a wire rack and let the muffins cool for 5 minutes. They will be soft and delicate immediately after baking.

Transfer the muffins to the wire rack and cool for at least 10 more minutes. The crumb will continue to set as the muffins cool.