Preheat oven to 400°. Place butter in a 12-in. cast-iron or other ovenproof skillet. Place skillet in oven until butter is melted, 1-2 minutes. Carefully tilt pan to coat bottom and sides with butter. Spread blueberries in bottom of pan. In a large bowl, whisk eggs, flour, milk, sugar, vanilla and cinnamon. Pour egg mixture over blueberries.
Bake until center is puffed and edges are browned, 30-35 minutes. Sprinkle with additional cinnamon if desired. Let stand 15 minutes before cutting.
