Preheat Your Oven and Skillet. Place a 10-inch cast iron skillet in the oven and preheat to 425°F. The hot pan is key to that big puff.
Make the Batter. In a blender (or bowl with a whisk), combine the eggs, flour, milk, sugar, vanilla, and salt. Blend until smooth. This should take about 30 seconds. Let the batter rest while the oven finishes heating.
Add Butter and Bake. Once the oven is hot, carefully remove the skillet and add the butter. Swirl to coat, then pour the batter directly into the hot pan. Immediately return it to the oven.
Bake and Watch It Puff. Bake for 18–20 minutes, or until the edges are golden brown and puffed up dramatically. Don’t open the oven door too early, or you’ll lose the rise!
Serve Immediately. Dutch Babies deflate quickly after baking. This is totally normal. Serve right away with your favorite toppings.