Preheat oven to 425 degrees F. Place a 9-inch cast iron skillet or pie pan into the oven to warm up.
In a large bowl, whisk eggs, milk, flour, sugar, vanilla and salt until well blended. You can also do this with an electric mixer or blender.
Remove hot skillet from oven. Add butter and, as it melts, swirl around the pan to coat. Pour in batter and return skillet to oven. Bake for 15-20 minutes, until golden brown and puffed.
Remove from oven. Top with desired toppings. Cut into slices and serve.