1. Preheat the Oven and Pan: Place a 10-inch cast iron skillet (or a similar oven-safe pan) into your oven. Preheat the oven to 425 F (220 C) with the pan inside. Allowing the pan to get very hot is crucial for the pancake to puff dramatically.

2. Combine Wet Ingredients: In a medium bowl, whisk together the sourdough discard, whole milk, eggs, granulated sugar, vanilla extract, and salt until well combined and smooth. The mixture should be fairly liquid.

3. Add Dry Ingredients: Gradually add the all-purpose flour to the wet ingredients, whisking continuously until just combined. Be careful not to overmix; a few small lumps are perfectly fine and will bake out. Overmixing can lead to a tougher pancake.

4. Melt the Butter: Once the oven and pan are hot, carefully remove the hot skillet from the oven using oven mitts. Add the 2 tablespoons of unsalted butter to the hot pan. The butter will melt almost instantly and sizzle. Swirl the pan to coat the bottom and sides evenly with the melted butter.

5. Pour and Bake: Immediately pour the pancake batter into the hot, buttered skillet. Carefully return the skillet to the preheated oven.

6. Bake Until Golden and Puffed: Bake for 20-25 minutes, or until the edges are deeply golden brown and have puffed up dramatically, while the center is set and lightly golden. Avoid opening the oven door during the first 15 minutes of baking, as this can cause the pancake to deflate.

7. Serve Immediately: Once baked, carefully remove the skillet from the oven. The Dutch baby will begin to deflate slightly as it cools, which is completely normal. Slice it into wedges and serve right away with your favorite toppings.