Preheat and prep: Heat your oven to 375°F (190°C). Place a 10-inch oven-safe skillet on the stovetop.
Toss the berries: In a bowl, mix blueberries with 1/2 cup sugar, lemon zest, lemon juice, and cornstarch. Stir gently to coat and set aside.
Melt the butter: Add butter to the skillet and melt over low heat.
Swirl to coat the bottom and sides. Turn off the heat but keep the butter in the skillet.
Make the batter: In a second bowl, whisk flour, baking powder, salt, 1/3 cup sugar, and cinnamon (if using). Add milk and vanilla.
Stir until just combined. A few small lumps are fine.
Assemble in the skillet: Pour the batter over the melted butter. Do not stir.
The butter should pool around the edges—that’s what creates crisp, caramelized bits.
Add the berries: Spoon the blueberry mixture evenly over the batter. Again, don’t stir. The batter will rise up around the fruit as it bakes.
Bake: Transfer the skillet to the oven and bake for 35–45 minutes, until the top is golden and set, and the berry juices are bubbling at the edges.
If using frozen berries, add 5 minutes as needed.
Rest and serve: Let the cobbler cool for 10–15 minutes to thicken. Scoop into bowls and top with generous scoops of vanilla ice cream.