Step 1: Preheat your oven to 425°F (220°C). Place two 6-inch cast iron skillets or individual oven-safe gratin dishes (or a single 10-inch skillet for a large Dutch baby) in the oven to get very hot while it preheats.
Step 2: In a medium bowl, whisk together the all-purpose flour and kosher salt. Set aside.
Step 3: In a separate larger bowl, whisk the eggs thoroughly. Add the milk, granulated sugar, and vanilla extract, whisking until well combined.
Step 4: Gradually add the dry ingredients to the wet ingredients, whisking until just smooth. Be careful not to overmix, as this can lead to a tougher pancake. A few small lumps are fine.
Step 5: Carefully remove the hot pans from the oven. Add 1 tablespoon of butter to each 6-inch pan (or 2 tablespoons to a 10-inch pan) and swirl to coat the bottom and sides as it melts. The butter should sizzle immediately.
Step 6: Immediately pour half of the batter into each 6-inch pan (or all into the 10-inch pan). Return the pans to the hot oven.
Step 7: Bake for 20-25 minutes, or until the pancakes are dramatically puffed, golden brown around the edges, and set in the center. Do not open the oven door during baking.
Step 8: Carefully remove the pancakes from the oven. They will begin to deflate almost immediately, which is normal. Serve immediately with your favorite toppings.