Preheat the oven to 400°F (204°C). Line a 12-cup cupcake pan with 6 liners, spacing them out so there is an empty cup in-between each liner. The extra airflow will help the muffins rise even taller.

Add the cooled, melted butter and brown sugar to a medium mixing bowl. Whisk until smooth, about 2 minutes. Then, mix in the egg, buttermilk, and vanilla extract. Add the all-purpose flour, baking powder, and Kosher salt to the wet ingredients. Whisk until it's about 90% combined.

Fold the mini chocolate chips into the batter just until there are no more streaks of flour. The batter will be a little lumpy, that's totally normal. 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 evenly divide the batter between the 6 muffin liners, filling them all the way to the top. Finish each muffin with a sprinkle of mini chocolate chips and 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 13-14 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!