Preheat your oven to 375°F (190°C) and generously grease a 9x13-inch baking dish with butter or non-stick cooking spray.
In a large mixing bowl, whisk together the all-purpose flour, granulated sugar, baking powder, baking soda, and salt until they are thoroughly combined.
In a separate medium bowl, whisk the buttermilk, eggs, melted butter, and vanilla extract together until the mixture is smooth and fully incorporated.
Pour the wet buttermilk mixture into the dry flour mixture and stir gently with a spatula until just combined; a few small lumps in the batter are okay. Be careful not to overmix. Gently fold in 1 1/2 cups of the fresh blueberries.
Pour the batter into the prepared baking dish and spread it into an even layer. Sprinkle the top evenly with the remaining 1/2 cup of blueberries and the packed light brown sugar.
Bake for 35-40 minutes, or until the top is golden brown, the edges are pulling away from the sides of the pan, and a toothpick inserted into the center comes out clean. Let the bake rest in the pan for 5-10 minutes before serving.
