Place butter into a cast iron skillet and place into oven to preheat skillet and melt butter.
In a bowl combine eggs, milk, sourdough starter, vanilla extract, and maple syrup. Whisk with a hand mixer for at least 5 minutes. Alternatively you can also combine in a blender.
Remove skillet from the oven and pour egg mixture into the pan. Place back into the oven. Bake for 20-25 minutes.