In a medium saucepan over medium heat, combine fresh blueberries, sugar, and lemon juice. Stir gently until the mixture begins to simmer, about 5 minutes. Mix cornstarch with a splash of water to create a slurry, then pour it into the bubbling blueberries while stirring. Cook for another 2 minutes until thickened, then remove from heat and fold in the remaining blueberries.

In a large bowl, whisk together flour, baking powder, baking soda, salt, and sugar until well combined. In another bowl, whisk the eggs, buttermilk, melted butter, and vanilla extract. Pour the wet ingredients into the dry mixture, stirring gently.

Preheat your oven to 375°F (190°C) and prepare a greased 9x13 inch baking dish. Pour half of the pancake batter into the dish, spreading it evenly. Spoon the blueberry filling over the batter, then pour the remaining pancake batter on top.

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

While the casserole bakes, whisk together powdered sugar, buttermilk, and vanilla extract for the glaze. Adjust buttermilk to your desired consistency.

Once baked, allow the casserole to cool for about 10 minutes. Serve warm with optional glaze, maple syrup, or toppings.