Preheat the oven to 425°F. Place an oven rack in the center position. Line a standard 12-cup muffin tin with paper liners.
In a large bowl, whisk together the all-purpose flour, granulated sugar, brown sugar, salt, and baking powder.
In a separate bowl or large measuring cup, whisk together the buttermilk, melted butter, eggs, and vanilla extract.
Pour the wet ingredients into the dry ingredients. Stir gently until only a few streaks of flour remain.
Reserve a small handful of mini chocolate chips for the tops. Fold the remaining chocolate chips into the muffin batter just until evenly distributed. Do not overmix.
Divide the batter evenly among all 12 muffin cups. An ice cream scoop works well for portioning the batter.
Sprinkle the reserved chocolate chips and a little coarse sugar over the top of each muffin.
Bake at 425°F for 5 minutes, then turn oven down to 400°F for about 15 more minutes. The muffins are done when the tops are golden brown, the centers spring back when lightly pressed, and a toothpick inserted into the center comes out clean or with a few moist crumbs.
Allow the muffins to cool in the muffin tin for five minutes. Transfer them to a wire rack and cool for at least 20 minutes before serving.