In a medium saucepan over medium heat, combine fresh blueberries, granulated sugar, and lemon juice. Stir gently until the sugar dissolves and the blueberries start to release their juices, about 5 minutes. Mix cornstarch with water, then add to the mixture and cook for another 2-3 minutes until thickened.

In a large mixing bowl, whisk together all-purpose flour, baking powder, baking soda, salt, and granulated sugar. In a separate bowl, beat the eggs, then add buttermilk, melted butter, and vanilla extract. Gradually combine wet and dry ingredients until just combined.

Preheat the oven to 375°F (190°C). Grease a 9x13 inch baking dish. Pour half of the pancake batter into the dish, then layer the blueberry filling on top. Pour the remaining batter over the blueberries.

Place the casserole in the preheated oven and bake for 30-35 minutes until the top is golden brown and a toothpick inserted comes out clean.

While baking, whisk together powdered sugar, buttermilk, and vanilla extract in a small bowl, adjusting the consistency with more buttermilk as needed.

Once baked, cool for about 10 minutes, drizzle with glaze if desired, and serve warm with maple syrup, whipped cream, or fresh mint.