Position rack to middle of oven and preheat to 450ºF (230ºC).
In a batter bowl or large bowl, add eggs, milk, vanilla, sugar, flour, cinnamon and salt in the order listed above, whisking after each item is added until the batter is smooth and there are no lumps. An immersion blender works well for this.
Add 2 tablespoons butter to a 10-inch cast iron skillet, place on middle rack in oven and set timer for 5 minutes. Remove skillet from oven, swirl butter around and pour in batter. Return skillet to oven and bake for 15 minutes until the pancake is fully puffed up and browned. *Do not open the oven or the Dutch baby won’t rise properly.
While the Dutch baby is baking, sauté apple slices in a small sauce pan with remaining tablespoon of butter until tender, about 5 minutes. Set aside.
Remove skillet from oven, top Dutch baby with sautéed apples and dust with confectioners’ sugar. Serve immediately.