Preheat the oven to 450 degrees . In a large (10- to 12-inch) cast-iron skillet, heat 2 tbsp. butter over medium heat until hot and bubbling but not brown. Add the berries, 1/4 cup sugar and 1 tsp. vanilla. Cook, stirring, until the berries are slightly softened, about 2 minutes. Transfer the berries to a bowl.
Wipe out the skillet and place it in the oven. In a small bowl, stir together the remaining 2 tbsp. sugar and the cinnamon.
In a medium bowl, whisk the eggs until pale and frothy, about 1 minute. Whisk in the milk, flour, salt and remaining 1/2 tsp. vanilla until smooth, about 1 minute (the batter will be thin).
Remove the skillet from the oven, add the remaining 4 tbsp. butter and swirl to coat the bottom of the pan. Pour in the batter and immediately return the skillet to the oven. Bake until puffed and golden-brown, 18 to 20 minutes.
Top with the berries and sprinkle with the cinnamon- sugar. Serve immediately.