How to Make It
Preheat oven 400°F. Whisk together flour, milk, water, eggs, and salt in a medium bowl until smooth. Set aside.
Spoon 1/2 tablespoon melted butter into each well of a 12-cup muffin pan, tilting pan to coat wells. Bake in preheated oven until pan is hot, 5 to 6 minutes. Quickly pour batter evenly into wells; do not stir.
Return to oven, and bake at 400°F until puffed and golden brown, about 20 minutes.