Preheat your oven to 375°F and line a standard 12-cup muffin tin with paper liners or grease the cups thoroughly.
In a large mixing bowl, whisk together the flour, granulated sugar, baking powder, baking soda, and salt until fully combined and free of lumps.
In a separate medium bowl, whisk the buttermilk, melted butter, eggs, and vanilla extract until the mixture is smooth and fully emulsified.
Pour the wet ingredients into the dry ingredients and use a spatula to gently fold them together until the batter is just moistened with a few streaks of flour remaining; do not overmix.
Gently fold in the raspberries until they are evenly distributed throughout the batter, being careful not to crush them.
Divide the batter evenly among the 12 prepared muffin cups, filling each nearly to the top, and sprinkle the coarse sugar evenly over each mound of batter.
Bake for 18 to 22 minutes, or until the muffins are golden brown and a toothpick inserted into the center of one comes out clean with only a few moist crumbs attached.
Let the muffins cool in the pan on a wire rack for 5 minutes before transferring them directly to the rack to cool completely.
