Place butter in a 12" cast iron skillet and set in a cold oven.
Preheat the oven to 425°F.
In the jar of a blender, add eggs, flour, and milk.
When the oven comes up to temperature, carefully take the pan out, swirl the butter around a bit, then pour the batter in.
Return pan to oven and bake for 20 minutes, or until nicely golden and puffed!
