Put your cast iron pan to the oven and preheat it to 230 C. You want the pan to get nice and hot so it will allow the pancake to grow nicely (similar method to making Yorkshire puddings).
Beat the eggs with sugar until fluffy and pale. Gently whisk in the remaining ingredients (apart from butter).
When your oven is hot, take the pan out with a pair of thick mittens (it will be extremely hot so be careful and have a trivet or wooden tray somewhere nearby if needed). Put the butter on the pan and swirl around to let it melt evenly. Once that's done, pour the batter in and quickly put it in the oven for about 20 minutes. Don't panic if it doesn't grow immediately- be patient and it will starting rising wonderfully.
Serve with maple syrup, fresh fruit or anything you like.