Preheat the oven to 425°F (220°C). Line a muffin tin with 6 paper liners or grease the muffin tin with butter or non-stick spray.

In a small saucepan, melt the butter over medium heat. Continue cooking until it turns golden brown and gives off a nutty aroma, about 5-7 minutes. Remove from heat and transfer to a heatproof bowl to cool slightly.

In a large mixing bowl, whisk together the flour, sugar, baking powder, baking soda, and salt.

In a separate bowl, whisk together the buttermilk, egg, and vanilla extract.

Pour the milk mixture and the brown butter into the dry ingredients. Stir until just combined, being careful not to overmix; it's okay if there are a few lumps.

Gently fold in the chocolate chunks until evenly distributed throughout the batter.

If time allows, let the batter rest for at least 30 minutes, up to 2 hours.

Divide the batter evenly among the prepared muffin cups, filling each about 2/3 full. Optionally sprinkle some sugar on top before baking.

Bake at 425°F (220°C) for 5 minutes, then lower the temperature to 350°F (180°C) and bake for an additional 14 to 16 minutes, until the tops are golden brown.

Remove the muffins from the oven and allow them to cool in the pan for a few minutes before transferring them to a wire rack to cool completely.