Preheat oven to 220°C.
It is very important that your ingredients are at room temperature. Begin by cracking the egg into a large bowl and whisk until frothy.
Add the milk, sugar and vanilla and whisk again for a moment until frothy again.
Combine the flour and salt into another small bowl and whisk into the batter. Set aside while you prepare the pans.
Rub the butter into two 6inch cast iron skillets or small oven proof dish. Completely covering the sides and bottom to prevent batter sticking.
Heat the cast iron pans over a high heat on the hob until the butter has melted and began to bubble. Tip in the batter and leave for a moment until small bubbles begin to appear around the outside. Transfer to the oven for 18 - 20 minutes until puffed up and golden brown.
Ensure oven has been preheated and batter is poured into a hot pan or these will not puff up as they should