Prep the oven and pan. Heat the oven to 375°F (190°C). Lightly butter a 9x13-inch baking dish or a deep 10-inch skillet.

Mix the blueberry filling. In a large bowl, combine blueberries, 1/2 cup granulated sugar, brown sugar, lemon juice, lemon zest, and cornstarch. Stir gently until the berries are evenly coated and glossy. Transfer to the prepared baking dish and spread into an even layer.

Make the topping. In a medium bowl, whisk flour, baking powder, salt, and cinnamon (if using). In a separate bowl, whisk melted butter, milk, vanilla, and 1/4 cup sugar. Pour the wet mixture into the dry and stir just until combined. The batter should be thick but spoonable.

Add the topping to the fruit. Drop spoonfuls of batter over the blueberries, leaving a few gaps for steam to escape. Don’t worry about covering every inch—those pockets create lovely crisp edges.

Finish and bake. Sprinkle the top with coarse sugar if you like. Bake for 35–45 minutes, until the topping is golden brown and a toothpick inserted into the biscuit portion comes out clean. The filling should be bubbling around the edges.

Rest before serving. Let the cobbler cool for at least 15 minutes so the juices thicken. Serve warm with ice cream or whipped cream.