Heat oven to 375°F. Place butter in a 9x13-inch dish to melt while preheating.
Toss blueberries with the first portion of sugar, cornstarch, lemon zest, and lemon juice. Rest 5 minutes.
Whisk flour, the second portion of sugar, baking powder, and salt in a bowl.
Pour in milk and vanilla. Whisk just until smooth and pourable.
Remove hot pan. Swirl melted butter to coat the bottom evenly.
Pour batter into the buttered pan. Do not stir.
Spoon blueberry mixture evenly over the batter. Do not stir.
Sprinkle cinnamon over the top. Bake 38–42 minutes until deep golden and bubbling.
Cool 10 minutes before serving. Enjoy warm, plain or with vanilla ice cream.