In a very large bowl stir together the blueberries, sugar, cinnamon, and lemon juice.
Stir in the cornstarch to coat the berries.
Pour the berry mixture into the prepared pan.
In a medium bowl whisk together the flour, 1/4 cup sugar, baking powder, and salt.
In a large liquid measure cup or bowl, whisk together the melted butter, egg, milk, and vanilla extract.
Make a well in the middle of the dry ingredients, then pour the wet ingredients into the well.
Stir the mixture together until combined - it will be thick.
Section off pieces of the biscuit dough (I usually do about ¼ cup) and flatten into circles about ½ inch thick. Cover the blueberry filling with the circles of biscuit dough. It doesn't need to completely cover the top of the berries.
Sprinkle the top with 1 tablespoon of sugar.
Bake the Cobbler
Cover the top of the pan with aluminum foil, and bake for 20 minutes.
Remove the aluminum foil and continue baking for 20-25 minutes or until the top is golden brown and you can see the blueberries bubbling underneath.
Remove from the oven and let cool for 15 minutes before serving.
*If using frozen berries, I recommend adding 1 additional tablespoon of cornstarch.