Preheat oven to 350°F (175°C) and grease a 9x13-inch baking dish.
In a large bowl, whisk together flour, sugar, baking powder, baking soda, and salt.
In another bowl, whisk buttermilk, eggs, melted butter, and vanilla until smooth.
Pour wet ingredients into the dry mixture and stir until just combined (do not overmix).
Gently fold in 1 cup of blueberries.
Spread the batter evenly into the prepared baking dish. Sprinkle remaining blueberries on top.
In a small bowl, mix brown sugar and cinnamon, then sprinkle evenly over the casserole.
Bake for 35–40 minutes or until golden brown and a toothpick comes out clean.
Cool slightly, dust with powdered sugar, and serve with maple syrup if desired.
Prep Time: 15 minutes | Cook Time: 40 minutes | Total Time: 55 minutes
Kcal: Approx. 250 per serving | Servings: 8