In a medium saucepan, combine 3 cups of fresh blueberries, granulated sugar, and lemon juice. Cook over medium heat, stirring occasionally, until the mixture simmers and the sugar dissolves, about 5-7 minutes. Mix cornstarch with a little water to create a slurry, then stir it into the blueberries to thicken. Cook for another 2-3 minutes until the filling is syrupy, then gently fold in the remaining blueberries. Set aside to cool.

In a large mixing bowl, whisk together all-purpose flour, baking powder, baking soda, salt, and granulated sugar until well-blended. In a separate bowl, mix large eggs, buttermilk, melted unsalted butter, and vanilla extract. Pour the wet mixture into the dry ingredients, gently stirring until just combined—be careful not to overmix, as a few lumps are fine.

Preheat your oven to 375°F (190°C) and grease a 9x13-inch baking dish with non-stick spray or butter. Pour half of the pancake batter into the prepared dish, spreading it evenly. Spoon the cooled blueberry filling over the batter, ensuring it's distributed well. Finally, pour the remaining pancake batter over the blueberries, covering them completely.

Place the baking dish in the preheated oven and bake the casserole for 30-35 minutes until the top is golden brown and a toothpick inserted in the center comes out clean. Once baked, remove from the oven and allow the casserole to cool for about 10 minutes before slicing.

While the casserole cools, whisk together powdered sugar, additional buttermilk, and a splash of vanilla extract in a small bowl to make the optional glaze. Adjust the consistency by adding more buttermilk if necessary.

Slice the Blueberry Buttermilk Pancake Casserole into generous squares and serve warm. Drizzle with the prepared glaze, and top with maple syrup, whipped cream, and fresh mint leaves.