Preheat Oven to 400 degrees Fahrenheit.
Add butter to a 12 inch cast iron skillet or similar baking dish and place in the oven for butter to begin melting.
Peel, core and thinly slice 3 medium apples and toss with cinnamon.
Add apple slices to cast iron skillet and allow apples to bake slightly while preparing the Dutch Baby Pancake batter. Stir apples at least once while baking to ensure they are evenly tender.
In a large bowl or blender mix together eggs, granulated sugar, salt, milk, vanilla, flour and melted butter until smooth. For smooth batter without a blender; incorporate the milk gradually to the rest of the mixture. Once flour is completely mixed with the rest of the ingredients, without clumps, then slowly incorporate more milk.
Once apple slices are slightly tender, pour over Dutch Baby Pancake batter and return to oven for 15 minutes.
During that time mix together Apple Crisp Crumble in a separate mixing bowl. Add brown sugar, oats, flour, cinnamon, salt, and diced cold butter. Use a pastry cutter to cut the butter into the oat mixture until mixture is approximately pea-sized crumbles. If you prefer, you could also use your hands or forks to cut butter into the mixture.
After Dutch Baby Pancake has baked for 15 minutes, add the Apple Crisp Crumbles to the top of the pancake, without taking the cast iron skillet completely out of the oven. (Use caution when doing this step.) Close oven and allow to bake for an additional 15 minutes.
Finished Cast Iron Skillet Apple Crisp Dutch Baby should look slightly puffy and crumbles should be golden when you pull it out of the oven.
Serve right away plain or with maple syrup, caramel syrup, milk, whipped cream or even ice cream.