Place butter in a 10-inch cast iron skillet and place in the oven as it preheats. As the butter melts, assemble your dutch baby ingredients.
Add flour, sugar, salt, milk, eggs and vanilla into blender and blend on high for 30 seconds until well-combined. Carefully pour the batter into the preheated skillet on top of the melted butter. Bake on the middle rack of the oven for 30 to 35 minutes or until the edges are puffed and brown. Serve immediately with your favorite toppings.