Preheat the oven to 400°F (204°C). Line a 12-cup muffin pan with 6 liners, spacing them out so there is an empty cup in-between each liner.

In a small bowl, use the tines of a fork to mash the banana until it's very smooth. You should end up with around 90-140 grams (⅓-½ cup) of mashed banana. Set aside.

In a medium mixing bowl, add the cooled melted butter, brown sugar, egg, and vanilla extract. Whisk the mixture until it's thick and lighter in color, 2-3 minutes. Then, whisk in the mashed banana until smooth. The mixture may look a little curdled, but it'll smooth out.

Add the all-purpose flour, baking powder, and salt. Using a rubber spatula, fold the dry ingredients into the wet ingredients until it's about 75% mixed. Then, add the milk, stirring just until the batter is combined and there are no more streaks of flour. The batter will be lumpy, that's ok! 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 raw sugar.

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 11-12 more minutes, until an inserted toothpick has just a few moist crumbs. Let the muffins cool in the pan for about 5 minutes. Then, transfer them to a wire rack to cool completely. Enjoy!