Preheat the oven to 425℉. Place the butter in the cast iron pan. Put the cast iron pan in the oven as it preheats to warm the pan and melt the butter.
While the butter melts and oven preheats, whisk together the flour, milk, eggs, discard, salt, and vanilla. Whisk until the batter is smooth, with minimal flour clumps.
Once the pan is hot and the butter melted, remove the hot pan from the oven. Pour the batter into the pan and return to the preheated oven. Bake for 20 minutes. Remove from oven and top with your favorite toppings (syrup, peanut butter, berries, whipped cream, cooked apples, etc.). Enjoy!