Prepare the Oven and Pan: 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. This step ensures that the muffins release easily after baking, preventing any sticking that could ruin their shape.

Mash the Bananas: 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. The more ripe the bananas, the easier they will mash and the better the flavor will be.

Mix the Wet Ingredients: 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. This mixture should be well-blended to ensure an even distribution of flavors in the muffins.

Combine the Dry Ingredients: 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. Properly mixing the dry ingredients helps the muffins rise uniformly and prevents any clumps of baking powder or soda.

Make the Batter: 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. Overmixing can lead to tough muffins, so stop as soon as the flour is incorporated.

Add the Chocolate Chips: 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. This ensures that the chocolate chips are well distributed without overworking the batter.

Fill the Muffin Cups: Divide the batter evenly among the 12 prepared cups. Each cup should be nearly full. Sprinkle the reserved chocolate chips over the tops. Filling the cups nearly to the top allows for a nice dome shape as they bake.

Start at High Heat: Bake at 425°F (220°C) for 5 minutes. Without opening the oven door, reduce the temperature to 350°F (175°C). This initial high heat helps the muffins rise quickly, creating a nice rounded top.

Finish Baking: 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 to get an accurate reading.

Check the Browning: 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. This prevents the tops from burning while ensuring the centers are fully cooked.

Cool in the Pan: 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, so this step allows them to firm up slightly for easier handling.

Finish Cooling: 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, resulting in a better texture when you take your first bite.