Add butter to a 12" cast iron skillet and place in the oven. preheat to 400°F
While the butter is melting, whisk the eggs.
Add flour to the eggs and continue whisking.
Then add in milk, sugar, cinnamon and salt. The batter can be mixed with a blender, hand mixer, or a simple hand whisk.
Once the butter starts to "bubble up" in the skillet, remove it from the oven and swirl it around to fully coat the bottom and a little up the sides.
Add the batter to your preheated skillet and place back in the oven for 18-20 minutes or until golden brown.
Notes
Be extra cautious when handling the hot skillet.
You can use a blender to blend smooth. Otherwise, try to get rid of most of the clumps when stirring. You don't have to get them all, you just don't want clumps of dry flour. It will bake just fine.
The batter is extremely runny. That is completely normal! Don't add more flour.