Preheat oven to 475°. Place cast iron skillet or oven-proof container with sides at least 2" high inside oven to heat at same time.
In a blender, combine eggs, flour, milk, and vanilla and blend until frothy, roughly 30 seconds.
When oven has reached 475°, melt butter in pan. Carefully pour batter into the pan and cook for 15–20 minutes, until sides have risen and top is gently browned. Remove from oven, place on a plate, and let cool for 3 or 4 minutes. Fill with your choice of toppings, including whipped cream and fresh fruit, bacon and maple syrup, jelly, jam, Nutella, or simply sprinkled with confectioner’s sugar.