In a medium saucepan over medium heat, combine fresh blueberries, granulated sugar, and lemon juice. Cook until the mixture begins to simmer, then stir in cornstarch mixed with cold water. Cook until thickened, about 2-3 minutes, then remove from heat and fold in remaining blueberries.

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

Preheat oven to 375°F (190°C) and grease a 9x13 inch baking dish. Pour half of the pancake batter into the dish, then spoon the blueberry filling over it. Pour the remaining batter on top, spreading gently to cover the blueberries.

Bake for 30-35 minutes until the top turns golden brown and a toothpick comes out clean. Allow to cool for about 10 minutes.

While cooling, whisk powdered sugar, buttermilk, and vanilla extract to make a glaze. Adjust consistency as needed.

Slice into squares, drizzle with glaze if desired, and serve warm with maple syrup and whipped cream.