Make sure one of the oven racks is set in the middle, then preheat the oven to 450ºF (230 °C).
In a large bowl, whisk together the flour, cornstarch, sugar, and salt.
In a large measuring cup, whisk together the milk, eggs, and vanilla extract.
Whisk one third of the wet mixture into the flour mixture until it's smooth, then slowly add the rest of the wet mixture, whisking until smooth.
Place the butter in a 10 inch cast iron skillet (see note!) and place it into the oven for 3 to 4 minutes or until the butter has melted and starts to sizzle.
Remove the cast iron skillet from the oven (watch out because the handle is burning hot so make sure to wear oven mitts) and pour the batter into the pan. Immediately return the skillet to the oven and bake for 16 to 20 minutes, until the edges are golden brown and crisp, and the pancake has puffed all around.
Remove the skillet from the oven and dust the Dutch Baby pancake with powdered sugar. Serve, cut into wedges, with berries, sliced citrus, bananas, whatever you like. Serve with maple syrup.
Notes
Though Sarah Kieffer recommends using a seasoned cast-iron pan, I used a non-stick Cuisinart fry pan that is ovenproof. This worked beautifully and every Dutch Baby I made in my fry pan rose tall and puffed beautifully. You can check out the pan I used on Amazon.