Preheat your oven to 350°F (175°C) and grease a 9x13-inch baking dish with cooking spray or butter.
In a large bowl, whisk together the flour, baking powder, baking soda, salt, and granulated sugar until well combined.
In another bowl, beat the eggs, then add the buttermilk, melted butter, and vanilla extract. Mix until smooth.
Pour the wet ingredients into the dry ingredients and stir gently until just combined. Be careful not to overmix; a few lumps are okay.
Gently fold in the blueberries, reserving a handful to sprinkle on top.
Pour the batter into the prepared baking dish and spread it evenly. Sprinkle the reserved blueberries on top.
Bake in the preheated oven for 25-30 minutes, or until the top is golden brown and a toothpick inserted in the center comes out clean.
Allow the casserole to cool for about 5 minutes before slicing. Serve warm, dusted with powdered sugar and drizzled with maple syrup if desired.