In a medium saucepan, combine 3 cups of fresh blueberries, ¼ cup granulated sugar, and 2 tablespoons of lemon juice. Cook over medium heat while stirring gently until the mixture starts to simmer and the sugar dissolves. This should take about 5 minutes; once simmering, remove from heat.

In a small bowl, whisk together 1 tablespoon of cornstarch with a little cold water to create a slurry. Slowly pour the cornstarch mixture into the simmering blueberry mix, stirring continuously. Cook for another 1-2 minutes until the mixture thickens and coats the back of a spoon. Once it reaches that perfect consistency, gently fold in the remaining 1 cup of fresh blueberries and set aside to cool slightly.

In a large mixing bowl, whisk together 2 cups of all-purpose flour, 4 teaspoons of baking powder, 1 teaspoon of baking soda, ½ teaspoon salt, and 2 tablespoons of sugar. Ensure the ingredients are well combined.

In a separate bowl, whisk together 2 large eggs, 2 cups of buttermilk, 4 tablespoons of melted unsalted butter, and 1 teaspoon of vanilla extract until smooth and creamy. Pour the wet ingredients into the dry mixture while gently folding them together until just combined.

Preheat your oven to 375°F (190°C) and grab a 9x13 inch baking dish. Generously grease the dish with melted butter.

Pour half of the prepared pancake batter into the greased baking dish, spreading it evenly across the bottom using a spatula.

Spoon the blueberry filling evenly over the layer of pancake batter.

Pour the remaining pancake batter over the blueberry filling, spreading it as evenly as possible.

Transfer the assembled casserole into the preheated oven and bake for 30-35 minutes.

While the casserole is baking, whisk together 1 cup of powdered sugar, 2-3 tablespoons of buttermilk, and ½ teaspoon of vanilla extract until smooth.

Once the casserole has cooled slightly, slice it into squares for serving. Drizzle each piece with the optional glaze.