Preheat oven to 425℉ with the cast iron skillet in it.
In a mixing bowl whisk eggs, milk, sugar, vanilla, and salt to incorporate air and add in flour. Stir until combined without over-mixing. Let rest for 10 minutes or overnight if making ahead.If using a blender add all ingredients (except butter) to blender and blend for 20-30 seconds and let rest as stated above. When the oven is preheated, carefully take out the skillet and put the butter in to melt. *Make sure to coat the bottom and sides of your skillet*
Give your batter a quick whisk and immediately pour your batter into the hot skillet and bake for 18-20 minutes until puffed and golden.
Remove and serve right away for best flavor and texture. *It is normal for it to deflate*