Preheat your oven to 425°F (220°C).
Place the butter in your cast iron skillet. Then put your pan in the pre-heating oven to melt the butter and prep the pan.
In a bowl, whisk together the flour, starter/discard, milk, eggs, baking soda, and salt until mixed.
When the oven has reached temperature, pour the batter into the skillet right away, and transfer it back into the preheated oven.
Bake for 20 minutes until the Dutch baby is puffed up and golden brown.
Serve immediately to watch the wonderful doubled puffy pancake deflate!
Add toppings of choice!
