Prepare Blueberry Filling: In a medium saucepan, combine 3 cups of fresh blueberries with sugar and lemon juice over medium heat. Cook for about 5-7 minutes or until the mixture simmers and the blueberries start to burst. Once bubbling, add a cornstarch slurry to thicken the filling, stirring consistently for another minute until glossy. Remove from heat and gently fold in any extra fresh blueberries, then let the mixture cool slightly.

Make Pancake Batter: In a large mixing bowl, whisk together the all-purpose flour, baking powder, baking soda, salt, and granulated sugar until well combined. In a separate bowl, mix the eggs, buttermilk, melted unsalted butter, and vanilla extract until blended. Pour the wet ingredients into the dry ingredients and stir gently, taking care not to overmix.

Assemble Casserole: Preheat your oven to 375°F (190°C) and grease a 9x13 inch baking dish. Pour half of the prepared pancake batter into the dish, spreading it evenly across the bottom. Next, layer the blueberry filling over the batter, ensuring it covers the entire surface. Finish by dolloping the remaining pancake batter on top.

Bake: Place the casserole in the preheated oven and bake for approximately 30 to 35 minutes until the top turns golden brown and a toothpick comes out clean.

Optional Glaze: While the casserole is baking, whisk together powdered sugar, a splash of buttermilk, and vanilla extract until smooth. Adjust the consistency if necessary.

Serve: Allow the casserole to cool for about 10 minutes before slicing it into squares. Drizzle with the optional glaze and serve warm with toppings like maple syrup or whipped cream.