Preheat oven to 425ºF (220ºC).
Reserving the butter, whisk together all remaining ingredients in a medium-sized mixing bowl until smooth and there are no lumps in the batter. A high-speed or immersion blender may also be used.
Place a 10-inch cast iron skillet in the oven on the middle rack and heat for 5 minutes. Remove skillet from oven and add butter, swirling it around until melted and it covers the bottom. Pour in batter and return skillet to oven to bake for about 15 minutes just until the edges are browned. *Do NOT open the oven or the Dutch baby won’t rise properly!
Remove from oven, add toppings and serve!