01 - Preheat oven to 375°F. Lightly grease a 9x9-inch baking dish with butter or cooking spray.
02 - In a large bowl, combine frozen blueberries, granulated sugar, cornstarch, lemon juice, ground cinnamon, and salt. Toss until berries are evenly coated. Spread mixture in the prepared baking dish.
03 - In a separate medium bowl, whisk together flour, sugar, baking powder, baking soda, and salt until well combined.
04 - Pour melted butter, milk, and vanilla extract into the dry ingredients. Stir gently until just combined; the batter will be thick. Do not overmix.
05 - Drop spoonfuls of batter evenly over the blueberry filling, covering most of the surface while leaving small gaps for steam and fruit bubbles to escape during baking.
06 - Bake for 40 to 45 minutes until the topping is golden brown and cooked through, and the berry filling is bubbling around the edges.
07 - Let cool for at least 15 minutes to allow the filling to set. Serve warm with vanilla ice cream or whipped cream if desired.