Put the butter in your chosen pan, and put it in the oven.
Preheat the oven to 425F.
In a blender (or a bowl with whisk) blend eggs, flour and milk.
When the oven comes up to temperature, 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!