Preheat oven to 400 degrees F.
Using a 10" cast iron skillet, melt the butter over medium heat. Add blueberries and remaining mixture ingredients.
Stir gently until cornstarch dissolves. Bring to a gentle boil and simmer for 3 minutes. Remove skillet from heat.
To prepare the topping, mix together flour, sugar, baking powder and salt in a medium bowl.
Add butter cubes to flour mixture. Use a pastry cutter to blend the butter in with the flour mixture. Add milk and vanilla extract. Stir with a large spoon until the batter is smooth
Drop spoonfuls of batter over blueberry mixture and gently spread over the whole top of the blueberries.
Bake for 20 minutes or until the topping is browned and the blueberry mixture is bubbling around the edges.
Add as much sugar needed to the Blueberry mixture depending on the sweetness of the blueberries.