In a saucepan over medium heat, combine 3 cups of fresh blueberries, sugar, and lemon juice. Cook while stirring occasionally for about 5-7 minutes, until the blueberries start to burst and the mixture simmers. In a small bowl, mix cornstarch with a little cold water, then stir it into the berry mixture for another minute until it thickens slightly. Remove from the heat and let it cool.

In a large mixing bowl, whisk together the dry ingredients: all-purpose flour, baking powder, baking soda, salt, and granulated sugar until evenly blended. In another bowl, mix the wet ingredients: eggs, buttermilk, melted unsalted butter, and vanilla extract. Add the wet mixture to the dry ingredients and gently combine until just mixed, leaving a few lumps for a fluffy texture.

Preheat your oven to 375°F (190°C). Grease a 9x13 inch baking dish with cooking spray or butter. Pour half of the pancake batter into the prepared dish, spreading it evenly. Next, layer the prepared blueberry filling over the batter, making sure to distribute it evenly. Finally, pour the remaining pancake batter over the blueberry layer, smoothing it out gently.

Place the assembled Blueberry Buttermilk Pancake Casserole in the preheated oven and bake for 30-35 minutes. You'll want to look for a golden brown top and check for doneness by inserting a toothpick; it should come out clean.

While the casserole is baking, you can make the optional glaze. In a small bowl, whisk together powdered sugar, a splash of buttermilk, and a few drops of vanilla extract until smooth. Adjust the consistency with more powdered sugar or buttermilk as needed to achieve a drizzle-able texture.

Once the casserole is baked and slightly cool, slice it into squares and drizzle the glaze over the top. Serve warm with maple syrup and whipped cream on the side, garnished with fresh mint leaves if desired.