Preheat the oven to 400°F (204°C). Line a 12-cup muffin pan with 6 liners, leaving a space in-between each liner.

Place the banana on a small plate. Use the tines of a fork to mash the banana until it's smooth and lump-free. You should have about 90-140 grams (⅓-½ cup) of mashed banana. Set it aside for later.

In a medium mixing bowl, add the cooled melted butter, brown sugar, egg, and vanilla extract. Whisk the mixture until it's thickened and lightened in color, 1-2 minutes. Then, mix in the mashed banana until smooth. The mixture may look a little curdled, but don't worry, it'll smooth out.

Add the flour, baking powder, and salt. Using a rubber spatula, fold the dry ingredients into the wet ingredients until the batter is about 75% mixed. Then, add the milk, stirring just until the batter comes together. Gently fold in the mini chocolate chips. Don't over-mix the batter! Optional: cover the bowl and let the batter rest for 20-30 minutes. This will help make the muffins extra tall.

Use a cupcake scoop to divide the batter between the 6 muffin liners, filling them all the way to the top. Finish each muffin with a generous sprinkle of mini chocolate chips.

Bake the muffins at 400°F (204°C) for 5 minutes. Then, reduce the heat to 350°F (177°C) and bake the muffins for another 11-12 minutes, until an inserted toothpick has just a few moist crumbs. Let the muffins cool in the pan for about 5 minutes and then transfer them to a wire rack to cool completely.