Preheat the oven to 375 degrees. Put the butter in a 9x13 glass baking dish and place in the oven to heat the pan and melt the butter.
Whisk the eggs and the milk together. Whisk in the flour, salt and cinnamon until mixture is smooth and thickened slightly. It will have the consistency of a pourable pancake batter. (Honestly, I dump all my ingredients in a blender at once and let the blender mix it for me!)
Remove the hot pan from the oven and swirl the melted butter around the pan to evenly coat it. Pour batter into the pan and return to the oven. Bake for 25-30 minutes, rotating the pan halfway through baking. The German pancake/Dutch Baby should be very puffed and very golden brown when you remove it from the oven.