Whisk together milk, egg, zest, salt until smooth. Whisk in flour until smooth.
Melt butter in a 9-10 inch oven-proof skillet over medium heat. Once melted and hot, pour in the batter.
Bake 20 minutes. Do not open the oven.
When cooked, it will be very puffy and the edges a dark, golden brown. Serve at once before it deflates. It’s fun to tear off chunks but you can cut it like a pie if you must have manners.
Serve with powdered sugar sprinkled on top, lemon wedges and/or jam