Preheat the oven to 425°F or gas mark 7 with the cast iron skillet inside.
In a food processor or blender add the eggs, milk, flour, vanilla, salt, and sugar and blend until batter is evenly smooth.
Once the oven is hot remove the cast iron skillet and add the butter. Make sure all of the butter has melted. Immediately pour the batter into the skillet and place back into the oven to bake for 25 minutes or until the sides have started turning dark brown.