Place a muffin pan on the rack, and heat the oven to 425ºF.
Place flour, milk, eggs, sugar, vanilla, and salt in a blender.
Blend for 1 minute - it will be very liquidy. Let the batter rest in the blender or processor for 10 minutes.
Carefully remove the preheated muffin pan from the oven. Add 1 piece of butter to each well. Pour the batter into the wells, about 1/3 cup per well.
Bake until the Dutch babies are puffed and lightly golden brown, 12 to 15 minutes.
Let cool for 1 minute before dusting with powdered sugar or adding fruit before serving.