Step 1: Preheat your oven to 425°F (220°C). Place a 10-inch cast iron skillet (or an oven-safe heavy-bottomed pan) with 4 tablespoons of unsalted butter directly into the oven as it preheats.
Step 2: While the oven and skillet are heating, prepare the batter. In a blender, combine 4 large room-temperature eggs, 1/2 cup whole milk (also room temperature), 1/2 cup all-purpose flour, 1 tablespoon granulated sugar, 1/2 teaspoon vanilla extract, and a pinch of salt. Blend on high until the mixture is completely smooth and airy, about 30-60 seconds. Alternatively, you can whisk vigorously by hand, ensuring no lumps remain.
Step 3: Carefully remove the hot skillet from the oven. The butter should be melted and sizzling, covering the bottom and sides of the pan. Swirl the butter around to coat thoroughly.
Step 4: Immediately and carefully pour the batter into the hot, buttered skillet. Don't stir or spread the batter; just let it settle naturally.
Step 5: Return the skillet to the hot oven and bake for 15-20 minutes, or until the Dutch baby is dramatically puffed, deeply golden brown around the edges, and set in the center. Avoid opening the oven door during the first 10-12 minutes.
Step 6: Once baked, remove the Dutch baby from the oven. It will begin to deflate almost immediately. Garnish with your favorite toppings right away and serve warm.