In a medium saucepan, combine 3 cups of fresh blueberries, ¼ cup of granulated sugar, and 2 tablespoons of lemon juice over medium heat. Cook for about 5 minutes until the sugar dissolves and blueberries begin to soften. Mix a tablespoon of cornstarch with a splash of water to create a slurry, then add it to the saucepan. Cook for an additional 2-3 minutes until thickened and fold in remaining blueberries, set aside to cool.

In a large mixing bowl, whisk together 2 cups of all-purpose flour, 1 tablespoon of baking powder, 1 teaspoon of baking soda, ½ teaspoon of salt, and ¼ cup of granulated sugar until well combined. In another bowl, whisk 3 large eggs, 2 cups of buttermilk, ½ cup of melted unsalted butter, and 1 teaspoon of vanilla extract. Gradually pour wet ingredients into the dry mixture, folding gently until just combined.

Preheat oven to 375°F (190°C). Grease a 9x13 inch baking dish. Pour half of the pancake batter into the dish, and spread evenly. Spoon the blueberry filling over. Pour remaining pancake batter on top, smoothing it out gently.

Bake for 30-35 minutes. The casserole should puff up and be golden brown on top. Check for doneness by inserting a toothpick; it should come out clean or with a few moist crumbs.

While baking, prepare an optional glaze by whisking 1 cup of powdered sugar, 2 tablespoons of buttermilk, and ½ teaspoon of vanilla extract. Adjust the consistency as needed.

Once baked, remove from the oven and let cool for about 10 minutes before slicing. Drizzle with glaze if using, and serve warm with maple syrup and whipped cream.