In a medium saucepan over medium heat, combine 3 cups of fresh blueberries, granulated sugar, and lemon juice. Cook, stirring occasionally, for about 5 minutes until the mixture simmers. Mix a cornstarch slurry with water, stir it into the berry mixture, and cook for another 2-3 minutes until thickened. Remove from heat and fold in reserved blueberries.

In a large mixing bowl, whisk together the dry ingredients: flour, baking powder, baking soda, salt, and sugar until blended. In another bowl, mix the wet ingredients: beaten eggs, buttermilk, melted butter, and vanilla. Pour the wet mixture into the dry ingredients, combining until just mixed; a few lumps are okay.

Preheat your oven to 375°F (190°C) and grease a 9x13-inch baking dish. Pour half of the pancake batter into the dish, spread it evenly, spoon the blueberry filling over it, and add the remaining batter on top.

Place the casserole in the oven and bake for 30-35 minutes until golden brown. To check doneness, insert a toothpick into the center; it should come out clean.

While baking, whisk together powdered sugar, buttermilk, and vanilla in a small bowl until smooth. Adjust thickness with more buttermilk if needed.

Once baked, let cool for 10 minutes before slicing. Drizzle glaze over top and serve warm with whipped cream, maple syrup, or fresh mint leaves.