Add a 12-inch cast iron skillet to the oven as it preheats so the skillet becomes hot.
To a blender, add the eggs, milk, flour and salt. Blend on medium speed until smooth.
If making by hand or with a hand mixer, whip the eggs well for about 1-2 minutes before adding the milk. Continue whipping as you add the flour and salt. Mix well until the mixture is smooth.
Add the butter to the hot skillet (remove the skillet from the oven and close the oven) and once it's melts pour the batter into the hot skillet.
Add the skillet back into the oven right away and bake for about 25-30 minutes. It's important not to open the oven while it bakes - you'll know it's done when the edges are golden brown and it's puffed up in places.
Serve immediately (this recipe makes 6 servings) with fresh berries and powdered sugar or maple syrup.