In another bowl, stir together flour, sugar, baking powder, and salt, then stir in milk until a batter forms.
1 cup all-purpose flour, 3/4 cup granulated sugar, 1 1/2 teaspoons baking powder, 1/2 teaspoon fine sea salt, 1/2 cup milk
Check to make sure the butter has melted in the baking dish and remove from the oven. Pour in blueberries in an even layer, then pour batter on top of the blueberries. It's OK if the batter does not completely cover the fruit; it will spread as it bakes and a rustic look is fine.
Stir together remaining sugar and cinnamon and sprinkle over the batter.
Place in the oven and bake for about 40 minutes, until juices are bubbly, cake is golden, and a toothpick inserted into the cake portion comes out free of batter.
Let cool for 10-15 minutes before serving to let juices settle and fruit cool.
Notes
Frozen blueberries can be used in place of fresh. No need to thaw.
If you are doubling this recipe to fit a 9x13-inch pan, use 8 tablespoons butter, not 12.