Place a 10” cast iron pan in your oven. Preheat the oven to 425F (with the skillet inside).
In a small bowl, whisk together the milk, vanilla, and eggs until combined.
Next, add the flour, cinnamon, and salt. Whisk until no lumps remain.
When the oven is preheated, remove the skillet (careful, it will be hot!). Add the butter to the hot skillet and swirl it around until it has melted and fully coats the bottom of the skillet.
Pour the batter into the middle of the skillet (all of the butter will be around the outside— that’s okay!) and place the skillet back into the oven.
Allow the dutch baby to bake for 12-15 minutes, or until it is puffed and golden brown around the edges.