The Kitchn: You'll flip for this Dutch baby casserole
1/2 teaspoon salt
1 teaspoon ground cinnamon
4 tablespoons unsalted butter, cut into 4 pieces
1 large apple, sliced (optional)
Place the flour, milk, eggs, 1/4 cup of the sugar, vanilla, and salt in a blender or food processor fitted with the blade attachment. Blend for 10 seconds, scrape down the sides, and then blend for another 10 seconds. The batter will be quite loose and liquidy.
Leave the batter in the blender and set aside to rest 20 to 25 minutes.
Meanwhile, arrange a rack in the middle of the oven. Place a 9-by-13-inch baking dish on the rack and heat to 425 F.
Whisk together the remaining 2 tablespoons sugar and cinnamon in a small bowl until evenly combined; set aside.
Using oven mitts, remove the baking dish from the oven and place it on the stove. Add the butter and tilt the dish to melt the butter and coat the bottom and sides of the baking dish. The butter will sputter and pop at first. Sprinkle 1/2 tablespoon of the cinnamon-sugar over the bottom of the baking dish. Pour the batter into the baking dish. Tilt the dish if needed so that the batter runs evenly to all sides. Sprinkle 1 tablespoon of the cinnamon-sugar over the batter.
Bake until the Dutch baby is puffed, lightly browned across the top, and darker brown on the sides and edges, 15 to 20 minutes.
Sprinkle with the remaining cinnamon-sugar, top with sliced apple, and serve warm.
Recipe notes: The Dutch baby will deflate quickly, but is still delicious. Store leftovers in the refrigerator for up to three days. Reheat in a 300 F oven until warmed through.