Preheat your oven to 350°F (175°C) and lightly grease a 9×13-inch baking dish or line it with parchment paper.
In a large mixing bowl, whisk together the all-purpose flour, granulated sugar, baking powder, baking soda, and salt until well combined.
In a separate bowl, beat the eggs until frothy, then add the buttermilk, melted and cooled unsalted butter, and vanilla extract. Whisk until smooth and well incorporated.
Pour the wet mixture into your dry ingredients and stir gently until just combined. Be careful not to overmix.
Gently fold in 1¼ cups of the blueberries, saving the remaining ¼ cup for the top.
Spread the batter evenly in your prepared baking dish and scatter the reserved blueberries over the top.
Sprinkle the coarse sugar over everything for a bakery-style finish.
Bake for about 40-45 minutes, or until a toothpick inserted in the center comes out clean with just a few moist crumbs. The top should be golden brown and slightly crackled.
Let it cool for about 10 minutes before serving.
