Spray a 9x13 pan with cooking spray. On the bottom, lay the fresh blueberries, and squeeze the juice of 1 lemon over the berries.
In a medium bowl, mix together the melted butter, milk, 1 cup sugar, salt, baking powder, flour, and vanilla. Pour over the blueberries, spreading evenly with a spatula (it doesn't have to be perfect, as the crust will bake over the berries).
In the same bowl, add the remaining ½ cup of sugar and 2 Tbsp cornstarch. Mix together with a fork and sprinkle on top of the cobbler mixture.
Over the entire cobbler, pour 2 cups of boiling water.
Bake for 1 hour. Serve warm with vanilla ice cream or fresh whipped cream.
Notes
If using frozen blueberries, you do not need to defrost them first. Reposted from March 2020.