Add the flour, sugar and salt to a medium mixing bowl and whisk to combine.
Add the milk, eggs and vanilla to a blender and blend until the eggs are frothy, about 1 minute. Next, add the flour mixture to the egg mixture and continue to blend until smooth, about 30 seconds.
Let the batter sit for about 15 to 20 minutes.
Place a medium cast iron pan (should be either a 9” or 10” skillet) in the oven. Heat the oven to 425ºF (220ºC) so the skillet heats as the oven heats.
Quickly remove the pan from the oven, add the butter and allow to melt, swirling it around the pan. Add the batter to the pan over the butter, tilting the pan to make sure it coats the bottom.
Put the skillet into the oven and bake until the Dutch baby pancake is puffed and golden brown around the edges, about 20 minutes.
For the apple topping:
While the Dutch baby cooks, heat a large skillet over medium heat then melt the butter. Add the brown sugar and stir to combine.
Add the apple slices, cinnamon and nutmeg, stirring to coat in the sugar. Cook until the apples are tender, about 6 to 8 minutes.
For serving:
Serve the Dutch baby hot in the pan topped with the sugared apples, whipped cream and confectioners sugar or cut into slices and top with the apples.